Is there any open source code for e-payment software, similar to PayPal?
If I understand the question properly, Aleksandr Blekh answers the question well here: Are there any actual open source payment gateways, not just a library that integrates with other payment gateways?
He mentioned three sources:
- Omnipay
- ActiveMerchant
- CI Merchant, GitHub: repo
Not entirely sure if this is what you are looking for