> For the complete documentation index, see [llms.txt](https://apidocs.akinon.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://apidocs.akinon.com/commerce-openapis/checkout/card-payment/select-installment-option.md). # Select Installment Option Sets the selected installment option for the credit card payment. **Validation**: * Selected installment must belong to the card's available installments * Installment must be active **Interest Calculation**: * Total amount is recalculated with selected installment's interest rate * Monthly payment amount is calculated (total / installment\_count) * Interest-free installments may be available based on campaigns **Selected Campaign/Rewards Reset**: * Selecting an installment clears previously selected campaigns * Clears previously selected rewards * Campaigns and rewards are re-evaluated based on new total with interest **Next Pages**: * `CreditCardConfirmationPage` ```json {"openapi":"3.1.0","info":{"title":"Overview","version":"1.0.0"},"tags":[{"name":"Card Payment","description":"# Checkout Card Payment\n\nAccept credit and debit card payments through a multi-step flow covering BIN lookup, installment selection, 3D Secure verification, payment confirmation, saved card selection, and order completion.\n\n## Authentication\n\nThese endpoints support both authenticated and anonymous sessions.\n\n- **Authenticated users**: Session cookie (`sessionid`) tracks checkout progress.\n- **Anonymous users**: Guest checkout is available when the `CAN_GUEST_PURCHASE` setting is enabled.\n## Request Format\n\nInclude the following header in all checkout requests:\n\n- **x-requested-with**: `XMLHttpRequest`\n\n## Dynamic Settings\n\nThe following settings are managed at runtime through the admin interface and do not require a server restart.\n\n### THREE_D_SECURE_ENABLED\nMaster switch for 3D Secure verification. When disabled, 3D Secure is skipped regardless of other rules.\n\n### THREE_D_SECURE_RULES\nRule-based conditions that determine when 3D Secure is required. Rules are evaluated in order; the first match triggers 3D Secure. Only active when `THREE_D_SECURE_ENABLED` is enabled.\n\nAvailable rule types: amount threshold, POS terminal identifier, failed attempt limit per email address, failed attempt limit per IP address, guest user status, first-time purchaser, payment currency.\n\n### INSTALLMENT_FILTERS\nFilters that restrict which installment options are presented. Filters are applied in sequence; each may remove options or stop further filtering.\n\n## Static Settings\n\nThe following settings are configured in the server environment and require a restart to take effect.\n\n### DEFAULT_CARD_SLUG\nFallback card identifier used when a BIN number is submitted but no matching record is found. Defaults to `other`."}],"servers":[{"description":"Akinon Checkout API","url":"https://{commerce_url}","variables":{"commerce_url":{"default":"sandbox.akinon.com"}}}],"paths":{"/orders/checkout/?page=InstallmentSelectionPage":{"post":{"tags":["Card Payment"],"operationId":"submitInstallmentSelection","summary":"Select Installment Option","description":"Sets the selected installment option for the credit card payment.\n\n**Validation**:\n- Selected installment must belong to the card's available installments\n- Installment must be active\n\n**Interest Calculation**:\n- Total amount is recalculated with selected installment's interest rate\n- Monthly payment amount is calculated (total / installment_count)\n- Interest-free installments may be available based on campaigns\n\n**Selected Campaign/Rewards Reset**:\n- Selecting an installment clears previously selected campaigns\n- Clears previously selected rewards\n- Campaigns and rewards are re-evaluated based on new total with interest\n\n**Next Pages**:\n- `CreditCardConfirmationPage`","parameters":[{"$ref":"#/components/parameters/AjaxRequestHeader"},{"$ref":"#/components/parameters/SessionCookieHeader"}],"requestBody":{"required":true,"content":{"application/x-www-form-urlencoded":{"schema":{"$ref":"#/components/schemas/InstallmentSelectionInput"}}}},"responses":{"200":{"description":"Installment selected successfully","content":{"application/json":{"schema":{"$ref":"#/components/schemas/CheckoutResponse"}}}}}}}},"components":{"parameters":{"AjaxRequestHeader":{"name":"x-requested-with","in":"header","required":true,"description":"Required header for AJAX requests. Must be set to `XMLHttpRequest` for all checkout requests.","schema":{"type":"string","enum":["XMLHttpRequest"],"default":"XMLHttpRequest"}},"SessionCookieHeader":{"name":"Cookie","in":"header","required":false,"description":"Session cookie header (e.g. `sessionid=abc123` or `osessionid=`)","schema":{"type":"string"}}},"schemas":{"InstallmentSelectionInput":{"type":"object","required":["installment"],"properties":{"installment":{"type":"integer","description":"Primary key of the selected installment option.\nMust be one of the installments available for the card.\nInstallment count of 1 represents single payment (no installment)."}}},"CheckoutResponse":{"type":"object","properties":{"context_list":{"type":"array","items":{"$ref":"#/components/schemas/PageContext"}},"pre_order":{"$ref":"#/components/schemas/PreOrder"},"errors":{"oneOf":[{"type":"object","additionalProperties":{"type":"array","items":{"type":"string"}}},{"type":"null"},{"type":"array","items":{"type":"string"}}]},"template_name":{"type":"string"}}},"PageContext":{"type":"object","properties":{"page_name":{"type":"string","description":"Class name of the page (e.g., \"IndexPage\", \"AddressSelectionPage\")"},"page_slug":{"type":"string","description":"URL-friendly slug for the page"},"page_context":{"type":"object","description":"Page-specific context data","additionalProperties":true}}},"PreOrder":{"type":"object","description":"Pre-order state containing basket, addresses, shipping, payment 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shipping option with calculated amount","properties":{"pk":{"type":"integer"},"name":{"type":"string"},"slug":{"type":"string"},"logo":{"oneOf":[{"type":"string","format":"uri"},{"type":"null"}]},"shipping_amount":{"type":"string","pattern":"^\\d+\\.\\d{2}$"},"description":{"oneOf":[{"type":"string"},{"type":"null"}]},"kwargs":{"type":"object","additionalProperties":true}}},"PaymentOptionDetail":{"type":"object","description":"Detailed payment option with type 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account for funds transfer payments","properties":{"pk":{"type":"integer"},"bank_name":{"type":"string","description":"Name of the bank"},"account_holder":{"type":"string","description":"Account holder name"},"iban":{"type":"string","description":"International Bank Account Number"},"currency":{"type":"string","description":"Currency code (e.g., \"USD\", \"EUR\", \"TRY\")"},"is_active":{"type":"boolean"}}},"PaymentChoice":{"type":"object","properties":{"value":{"type":"string"},"label":{"type":"string"},"price":{"type":"string","pattern":"^\\d+\\.\\d{2}$"}}},"DataSourceShippingOptionDetail":{"type":"object","description":"Data source shipping option for multi-vendor shipping","properties":{"pk":{"type":"integer"},"shipping_amount":{"type":"string","pattern":"^\\d+\\.\\d{2}$"},"shipping_option_name":{"type":"string"},"shipping_option_logo":{"oneOf":[{"type":"string","format":"uri"},{"type":"null"}]},"data_source":{"type":"object","properties":{"pk":{"type":"integer"},"title":{"type":"string"}}},"description":{"oneOf":[{"type":"string"},{"type":"null"}]}}},"PreOrderAttributeBasedShippingOption":{"type":"object","description":"Attribute-based shipping option selected for a product group","properties":{"pk":{"type":"integer"},"shipping_option_name":{"type":"string"},"shipping_option_logo":{"oneOf":[{"type":"string","format":"uri"},{"type":"null"}]},"shipping_amount":{"type":"string","pattern":"^\\d+\\.\\d{2}$"},"product_ids":{"type":"array","items":{"type":"integer"}},"attribute_value":{"type":"string"},"attribute_key":{"oneOf":[{"type":"array","items":{"type":"string"}},{"type":"null"}]}}}}}} ``` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://apidocs.akinon.com/commerce-openapis/checkout/card-payment/select-installment-option.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. 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