The activation journey has always been biased towards goodybags, and it's not particularly clear how to only add airtime credit when paying be credit or debit card. The journey has also changed recently so that those using a voucher can get a goodybag during activation and redeem the voucher to pay, rather than the old way of adding credit before exchanging it for a goodybag.
This makes me wonder whether the cries for help come from people after the activation topup, or after a subsequent topup.
The thing is giffgaff do need to do something to minimize the issue. If new members knew that goodybags and airtime credit are 2 different things, then maybe it would mean that giffgaff would save money by reducing the number of refunds they give.
I think it's the word topup that's confusing. Many times we see "I topped up my goodybag with another £10". We can guess immediately that they actually bought £10 of credit. But they're using the word topup thinking it means "add something to whatever I already have".
Giffgaff aside, technically they're right. You can't top up something you don't already have. If you had no water jug, you would not say "I need to top up my water jug" you would just say "I need some water".
So if somebody has a goodybag and has not previously bought credit, then pedantically the only thing that they can top up is their goodybag. And remember, new members these days are no longer given the option to buy credit along with their first goodybag (unless they buy a £5 goodybag with a £10 voucher). So, most new members will have only a goodybag when they first join.
Of course, we as established members know you can't top up a goodybag, but a new member with a goodybag that's running low will come to the main menu and see that topup option and will presume its their so that they can top up the only thing they have, which is a goodybag.
Somewhere else, I suggested a possible solution to this: we merge the menu links to a single "top up or buy goodybag" link. That goes to the present buy page https://giffgaff.com/buywhich already says what credit is for as well as explaining goodbags and gigabags. Yes, the explanations could be improved but the principle is there -- the main banner link should go to what wikipedia calls a "disambiguation" page -- one that helps you make the right choice from several possibilities.