It's always worth disabling mobile date prior to topping up your phone. This is because the moment that credit hits your phone, apps will automatically beging working in the background if data is left on.
That said, agents are usually more than willing to rectify the issue and I'm sure will be happy to process the £25 goodybag for you. You mentioned you've already been in touch with an agent- did you contact an agent via this link? https://support2.giffgaff.com/app/ask/Top-up-balances-and-goodybags/Credit-debit-card-or-PayPal-over...
In either case, please note that agents can take upto 24 hours to reply, but you should normally hear back from them within a few hours. Agents work until 11pm.
Agent replies can be found here: https://giffgaff.com/support/questions
I hope this helps! Please let us know how you get on.