Hi there, If you just did it by mistake then they don't have to give you the money back I am afraid.
It would be best for you to just leave it on your account and use it next time.
Remember your £10 topup NEVER EXPIRES. You can use it even in a years time (as long as you are still with giffgaff). It does not run out. You can also spend it on your next goodybag if you want (on the payment page when you buy a bag next month the first button will be "spend your existing credit topup".