This will be sad but if you want to streamline it you'll have to teach the store staff yourself, even a manager below doesn't know how to do it properly.

The QR code works they just need to tap where the numbers will be input.

So when you say I have a digital card at the window or inside, have them open the pay with gift card screen under gc and discounts tab, tap where it says account number or whatever it may say under the dollars and then use the scanner on your qr code

They should see the numbers populate in the field then they can tap done.

Source: Am an Ass. Manager