To clarify, if you are looking for someone to build a payment gateway for the payment processor, only approved partners are able to do so and only for payment gateways that have been invited to join the platform.
If you are looking to customize the checkout experience on a Shopify Plus store, that is different and definitely doable! You can adjust the checkout flow on PLUS store accounts that have access to the checkout.liquid.
If you have additional questions about this please let me know.
Shay | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog