Open and scan
Tap the card tile on your home screen. ShowCards raises your screen brightness to maximum and renders the barcode pixel-perfect, which is exactly what checkout scanners need to read it in one pass.
If the scanner does not beep
Some older laser scanners struggle with any phone screen. Hold the phone a little further away and flat under the beam. If it still fails, show the cashier the number below the barcode so they can key it in.