I want Joe Public to be able to turn up at a public (or flight school accessible) computer terminal, tap in a credit card number for a flight test from my shopping cart, and walk away knowing that they paid.
Straight forward, simple, click click, tap, tap - "enter", done.
The system I have just now (VirtueMart) seems to require that the 'customer' become a registered member of the site (as you and I are with PPRuNe) before they can pay.
Eyes filling with tears now.....and I've got a flat bit on me 'ead. (Wall still OK though).