Welcome to the community and great question! The payment code is a feature that allows a store to scan a QR code on your mobile phone or a payment code is displayed on the phone that is then entered into the register key pad.
This QR code or payment code may be displayed in the PayPal mobile app or it could also be in the merchant's specific mobile app. If you're looking in the PayPal mobile app, there should be a list of merchants displayed under the "Shop" option. Then there would be an "Order" button next to the merchant.
I hope this helps!
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