Paysera Symfony2 Integration

It's a common need to integrate a payment gateway into a project. Even though there are many well known systems and aggregators like PayPal, RBKMoney, Paymentwall, Robokassa and so on, I want to talk about Paysera. It's another quite new payment system. They claim to have low commission fees for a convenient and wide range of services. Paysera allows your users to pay via cards and SMS. Integration is quite easy, but has some unclear moments during the process that I want to point out.