It has to be used within about 3 or 4 months I think on (normally) anything digital.I've heard this before but never tried it. Doesn't the credit go towards a digital download and have to be used within a set period of time? I didn't think it could be kept and saved towards Prime membership.
So if you regularly buy ebooks for your kindle it ends up knocking money off them or paying for them in full, it's rare for me to pay anything like the full price for my books

I think I got about £50 worth of credit over Christmas alone a couple of years back (they sometimes up it to £3 a time when they're busy)..
Of course this only works if you are going to buy your books/mp3's from amazon anyway.