Hello, there are two ways to do this. Connection of payment systems one by one. This is a long and difficult path that you will have to go through on your own. For each system, you need to find a partner bank, prepare all the necessary documents and hire developers who integrate the system into the site. If an error occurs on the bank's side during the online payment process, the transaction will be rejected, and you will lose the client and his money. Connection of several payment systems at once through the gateway. In this case, most integration concerns lie down with a special service - the gateway. He cooperates with many banks, which saves the seller from having to prepare documents for each of them. In addition, if an error occurs while processing the transaction, you will not lose the client: the gateway will automatically redirect the payment through the reserve bank. By the way, I can recommend you to visit https://payadmit.com/, where there is information about one of the most reliable payment gateways.