Creating your own Monero payment gateway is very easy, here are the main steps:

1. Run monero-wallet-rpc --tx-notify "script.sh %s", where %s is txid, which will be automatically passed to your script as the first argument. Your script will be executed with each received transaction.

2. When a buyer, for example in your online store, wants to pay for a product, the site should give him an address to which the money will be transferred. To do this, your web application must perform an RPC request make_integrated_address to the daemon you launched to receive such an address integrated_address and payment_id. The address is given to the buyer, and the payment identifier is saved for future verification.

3. After the money is transferred, your script from the first point will receive txid which must be passed to your web application. It will receive payment_id and amount using the RPC method get_transfer_by_txid — the identifier and amount of the payment.

4. The application checks the payment ID and the received amount, which is measured in atomic units 1 XMR = 1e12 (atomic-units). Keep this in mind when comparing the amount, it is written at the beginning of the documentation.

Making one-time payments without an internal balance is very inconvenient, so it is best to create a user balance in the database on the site, which can be replenished at any time.