Tiny numbers. Long codes. One mistake.
The print is too small
Thermal voucher slips fade fast. The digits are tiny. For older eyes and low vision, they're unreadable.
The codes are too long
A 12 to 16 digit PIN, plus a different USSD string for every network. Easy to get wrong.
So people ask for help
Every time they buy a voucher, they hand their phone to someone else. That's the dependency Airsnap removes.