Getting money from a user is EXTREMELY hard, especially if you are outside US.
If you want frictionless payments, you should probably get a merchant account for credit cards, you cant get CC info to Appengine, for that reason you have to use a 3rd party service to handle payments etc. (Spreedly for example)
There are services like Stripe etc. but they are US only. You may check out Paypal but all useful features of Paypal are US, UK, AU only etc.
So from your use of English I guess you are not from US, your best bet is to find an intermediary payment processor that will handle payments etc. for you. I would suggest
http://www.paymentwall.com/ . There is some friction, but certainly its easy to use.
I would also like any insight on payments, especially, is there something like Stripe for non-US entities?