The customer has detailed technical specifications. In this case we can determine the cost and the deadline right away. The payment is usually proceeded in two stages: before the beginning of work a non refundable advance in the amount of 20-100 % is made, after works completion the rest of the payment is made. The amount of advance depends on the work scope.
But usually there is no prepared technical specification, there is only some vague vision what problems need to be solved. In this case the customer pays for actual hours worked. That is 30 € per 1 hour. The customer pays for the first three hours of work in advance. If the actual hours worked turn out to be fewer than it is supposed to be, the rest of the money is nonrefundable. When the project lasts for a very short period, the bill is presented after works completion. But if the project has a long run, the bill is issued every week.
The payment can be made via bank transfer or with PayPal.