A good idea can be to top-up with the initial £10 and when your free £5 gets added, within 24 hours of activation, buy the £12 goodybag that you want.
That will leave £3 credit on your account to cover the cost of any "out of goodybag" charges. To listen to voicemail, International calls, 0870, 0845 etc. Always handy to have some spare credit.
Many members offer free First Class delivery to speed things along. Giffgaff delivery can be quite long,as much as two weeks in some cases. Check the posts complaining of that.
Obviously that requires you giving the member your name & address to deliver to. If you would rather not do that, just click on an affiliate link in someone post signature.