No app, nothing to print. A code that gets exactly one person in, once.
Buy
Pick a ticket, pay by card. No account needed - a guest checkout works the same as a signed-in one.
Get your link
Straight after paying, and by email once that is switched on, a private link opens your tickets. Nobody else's.
Show your QR
Each ticket is its own code. Screenshot it, save the link, whatever gets it on your phone at the door.
One scan, in
The first scan admits. Every copy after it - a screenshot passed on, a second phone - is refused.
A cracked screen, a bad connection, a dead battery at the wrong moment - every ticket also carries a short code printed under the QR. Door staff can type that in by hand and it gets you in exactly the same way.
Your ticket link opens your tickets for anyone who has it, the same way a paper ticket would. Do not post it anywhere public.
Transfer it to someone at exactly what you paid, or hold onto it for resale once that opens. How fair resale works.