I've just recently noticed (while Christmas shopping) that Amazon UK are now offering Prime members a promotional credit if they choose slower ("No-Rush") shipping for an order instead of next day delivery.
I've got £9 to spend on ebooks with this from my Christmas shopping. Well, £6.50 now, since I've already used some.
There are still some rough edges. There's no way (at the moment) to see how much promotional credit you have left in your account (I work it out from the emails I've received). The credit does expire after a couple of months if you don't use it.
But I'm very happy. It may well be that I'll get most of my prime membership back this year in promotional credits for ebooks!
Details of the scheme are
here. I've no idea if its a permanent benefit, or a seasonal promotion.