Hi, as you're a new customer
an agent may refund all your lost credit as a one off goodwill gestureHttp://giffgaff.com/buy
Click the goodybag section and select a goodybag.
On the ways to pay, there's an option to use a voucher
Input your voucher code and click the yellow continue button
additional points when using a browser, you need to remove your debit card details or add your PayPal details and click to use that as your default payment, otherwise the use voucher option won't appear
(via the my giffgaff app)
DON'T click the buy goodybag button from the main account page
Click the menu button (top /left)
Click "ways to buy"
You'll then be presented with the same pages as though you were using a browser, and the pay by voucher option will be visible