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Payum/Payum
PHP Payment processing library. It offers everything you need to work with payments: Credit card & offsite purchasing, subscriptions, payouts etc.
pronamic/wp-pronamic-pay
The Pronamic Pay plugin allows you to easily accept payments with payment methods like credit card, iDEAL, Bancontact and Sofort through a variety of payment providers on your WordPress website.
pronamic/wp-pay-core
Core components for the WordPress payment processing library. This library is used in the WordPress plugin Pronamic Pay: https://www.pronamicpay.com/, but also allows other plugin developers to set up a payment plugin.