If still having problems buying a phone please contact giffgaff agent for them to look into this further. To contact a giffgaff agent please go to your My giffgaff page and you'll find links to the Ask an Agent form half way down the left hand side. Here's a https://support2.giffgaff.com/app/ask/Phones/Buying-a-phone/form/ link to it in case you're on a mobile.
please note as giffgaff agents deal with a great deal of enquires a agent can take up to 24 hours to answer your question. Any agent replies can be found in the “Messages From Agents“ section here https://giffgaff.com/support/questions