How can i make my own services that will work like Google CheckOut

Hello Experts,
I am looking for some urgent help. We are working on an e-commerce website for one of our client. Now, when a customer placed an order, we have given an optional link for Google CheckOut functionality for online payment.
Now the problem is:  client has interested to make his own web-services that will work like Google checkout feature. Actually, the client has a three to four independent e-commerce based website. And he wants to implement his own Google ChedkOut like functionality on these website. Is any expert having any idea about how can we achieve this task. Also Is there any idea about architecural design of google checkout and its server side functionality like: on the server side what is the database design and how the data process over the server.