Wrote Topped up £10, instantly dropped to £9.99.
So there is no way data could be used.
if that’s the case there clearly is a 1p glitch and as you say is different to expired goodybag,
its hard to tell sometimes as getting full facts as they happen can be an almost impossible task ,
a friend who was clipped 1p after trying to use the same method on my recommendation
I'm guessing that despite using the 43430 service, the friend followed the option 1 method to top up from a voucher instead of the option 2 method that would have allowed them to buy the goodybag direct.
Its unfortunate that option 1 comes before option 2 for historical reasons. Sadly, too many people don't understand the difference between topping up and buying a goodybag and therefore follow that first option even though it's not the option that they need.
Maybe it would help to change "press 1" to "press 3" and not mention it until after option 2.
there is a data charge despite giffgaff protestations that their website is free
If payment is by credit or debit card, then it may involve an authorisation on a site such as Verified by Visa
Unlike paypal, the VV site is not currently free rated -- which has two impacts:
Getting VV and other verification servers free rated might help solve this problem -- you can support that idea at https://labs.giffgaff.com/idea/16709244/sites-providing-payment-verification-services-should-be-reac...