KhadashPay
The KhadashPay project is an open-source payment system.
...As for the customers of the store where the KhadashPay is deployed, Khadash pay doesn't keep any logs whatsoever and doesn't require a bit of your personal information to create an account.
You can find the tutorials here:
V2.0 (ESP32 Version): https://www.instructables.com/KhadashPay-V20/
V2.0 (Raspberry Pi Pico Version): https://www.instructables.com/KhadashPay-V20-Raspberry-Pi-Pico-Version/
V3.0 (STM32F401CCU6 Version): https://www.instructables.com/KhadashPay-V30-STM32F401CCU6-Version/
V3.0 (ESP32 Version): https://www.instructables.com/KhadashPay-V30/
V3.5 (ESP32 Version): https://www.instructables.com/KhadashPay-V35/
The version of the KhadashPay that stores the data in the cloud and allows you to operate several payment terminals is available on https://sourceforge.net/projects/khadashpay-firebase-edition/