Invoice settings hold the merchant-level defaults applied when creating and calculating invoices.

These include the default due date, tax percentage, discount rate, shipping fee, and processing fees.
Settings also record the merchant's chosen payment method type and payment processors for invoices.
A merchant can toggle notifications, permissions, and other behaviors, such as staff discount authority.

The Invoice Settings endpoints support retrieving and updating a merchant's settings.
Both endpoints are scoped to a single merchant, identified by merchantId in the path.
Retrieving settings returns the current defaults; updating them requires resending the complete set of fields.
Unlike Configurations, these values are set per merchant rather than returned as fixed platform defaults.

* GET /pay-int-api/merchants/{merchantId}/invoices/settings.
Retrieves a merchant's invoice settings.
* PUT /pay-int-api/merchants/{merchantId}/invoices/settings.
Updates a merchant's invoice settings.