Shopify checkout API development

I'm looking for a skilled freelancer to develop an API for our Shopify store checkout for gift card only. Specifically, we need it to integrate with the Shopify card vault for payment processing, while also including custom payment gateways, shipping rate calculators, and discount code integration. we do not need page we just need to checkout using api might be developed in postman, and need the project done in less than a week.