Support
Printing the 4 × 6 sheet
Save the print sheet from the app and order a standard 4 × 6 inch (10 × 15 cm) print at any photo counter or print service. Choose the option that prints the whole image without cropping or "fit to frame", then cut along the lines. Check one copy with a ruler: a US photo is exactly 2 inches square, a UK or Schengen photo 35 mm wide.
The background looks slightly different from my wall
That is expected. Stamped replaces the background with the colour the rules ask for. You still get the best edges around your hair with a plain, light wall behind you and even light from the front.
Restoring a purchase on a new phone
Open Settings in the app and tap "Restore purchases". Unused unlocks come back. Your photos themselves are stored only on the phone that made them, so save them to your camera roll; they are not restored to a new phone.
Refunds
Apple handles refunds for App Store purchases at reportaproblem.apple.com. If a photo was rejected, please also tell us below what the reason was.