Is there any open source code for e-payment software, similar to PayPal?

[chackerian]

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:

1. Omnipay
2. ActiveMerchant
3. CI Merchant, GitHub: repo
   Not entirely sure if this is what you are looking for