Actually I've looked deeper at package this night. The Package is only responsible for some kind of real time transaction protocol.
How it works:
1. We prepare transaction data (purchased products, price for each product, total price, .... etc.) to epay.
2. Data submitted to epay.
3. Epay takes care of all data.
3. Epay respond (Y,N) transaction accepted or not accepted.
4. We check transaction status.
5. Epay respond for status (Pay, Wait, Deny, Error) of transaction.
Thats all with some little exceptions ;)