The simplest answer is that this is not typically an option for sales channel apps. From our API terms:
not use an alternative to Shopify Checkout for web checkout or payment processing, or register any transactions through the Shopify API in connection with such activity, without Shopify’s express written authorization;
However, when you’re working with marketplaces looking to integrate, the payment processing request form in your partner dashboard (screenshot I shared earlier) is how you can kick off a conversation with our partnerships team to discuss options further.