Talking of credits applied randomly here's how they run Audible.
I participated in the offer buy three audio books and get a £10 coupon. Coupon duly arrived and I selected a book costing £9.35. I put book in my basket selected checkout and the price jumped to £10.60.
I contacted Audible who apologised and added another £10 coupon so I wasn't out of pocket. I purchased the book and another one, the two books costing less than £20.
I then returned one of the two books as couldn't stand the narrator (sometimes it takes longer than the sample to realise this). I got a coupon returned to my account.
I go to use the coupon and my credit card gets charged. I enquire why. The answer, it's a mistake but we'll refund you and debit the coupon, but please have another £10 coupon as compensation, ohh and here's a free credit too.
So my £10 coupon ended up costing Audible £40.
I dare not query it again, who knows what they'll give me.