Debit/Credit card payment problems can be caused by the following issues.
Still no luck then If you have previously stored your card details (https://giffgaff.com/profile/payment-details) then delete, reinput and try again as this has been known to cure some problems (especially with the 3 digit security code/CVV). You can also do this by clicking 'use a different card' and re-entering the card details (even if you want to pay with the same card) during the check out process.
Error "We haven't been able to process your payment due to an issue with your giffgaff account. Please contact a giffgaff agent." then try this:
If still unsuccessful or you have multiple accounts then raise a case with the agents https://giffgaff.com/support/ask/cant_topup_airtime giving the following information so the team can look into your case as quickly as possible.
--> The last 4 digits of the card you are trying to use.
--> A description of what you have been trying to do or have experienced, along with any error messaging.
--> If multiple accounts are being used, the usernames, numbers of the giffgaff accounts you are trying to top up using this card. If you have other 02 or Tesco mobile accounts include this information.
--> The postcode that the card you are trying to use is registered to.
Meanwhile you can top up your account by buying a voucher (giffgaff http://bit.ly/giffgaffvoucher or O2 will also work) and redeeming it (https://giffgaff.com/buy/redeem). You can then buy a goodybag/gigabag if required from the redeemed credit Buying goodybag/gigabag from credit
Note: turn off your data on the phone until you have a live goodybag/gigabag to avoid losing credit due to PAYG data usage.
It is worth noting that if your card expires at the end of the month that no error message seems to be displayed on the Verifed By Visa page, the form simply submits without doing anything. Very confusing! When I added my new card in the top up screen all then worked fine.