Hi, can you please tell me which payment gateway are you going to use?
I have +5 years experience as web developer and I coded a lot of custom payment gateways for Woocommerce, Magento, Opencart, custom shopping carts etc integrating them with Paypal, Skrill, Bitcoin and other payment gateways incluging local banks and other online services.
If you have any question just let me know.
Greetings!