There are guides for sole payment:
The main differences:
The Flex.API is small and simple, but you need to implement the payment method selection.
Our Smart Checkout provides a ready-to-use checkout solution.
Our secuconnect API can also be used to implement your custom checkout solution incl. recurring payments, one-click checkout, and much more.
Our Smart Checkout can also be used for purchase orders, where the checkout process also includes the delivery:
Here is a comparison of the different website integration options: Integration Patterns Smart Checkout.