Zynga and Facebook’s joint statement: “Facebook and Zynga have transitioned Facebook Credits to be the exclusive method of payment for most Zynga network games on the Facebook platform.

Even if you don't receive a statement, you're still responsible to make that payment on time.

The confirmed drawing, demanding indent, shipping documents, payment certificate, quality statement, to and from formal documents, and so on all form the valid subsidiary file of this contract.