Hi @navvy’s point (ie it would advise people of the amount to be taken) and resolve the possible problems with your suggestion for a 10 day credit period AFTER purchase, why not give people a 10 day period BEFORE purchase.
They could be sent an e-mail and/or text saying:
"Your recurring bag paid by DD is coming up for renewal.
We will be taking £[insert amount here] pounds from your account on [insert date here].
If you are happy with this you do not need to do anything.
However if you wish to change your bag to a different one you MUST do it before [insert date here]."
One possible complaint that users could make is that this would not be as flexible as the present system (ie they can currently change their bag quite close to it's renewal date) but as long as the gap is not too long then it is no different from what happens now where you cannot change your bag because it is going through the update process.
Indeed this might help giffgaff manage that process better in that they could prioritise those bags that are paid by DD and do them first in which case they could also add a time to the deadline date stated in the advisory message.
I have a variable DD with my landline provider which also offered a discount for early payment. If I tried to make this payment once the DD cycle had begun I was advised that changes were not possible so this could also be introduced?
Doh! I'm on holiday and promised myself I wouldn't contribute while away (although I've been reading forums most days, albeit not logged in) and now I'm breaking that promise. But I feel that this is an important thing to get right (otherwise there'll be LOADS of posts on H&S) so hope my idea provides food for thought.