I was at a Murphy Oil gas station today and noticed on a sign that they have a promotion offering $5 off any retail gift card. Among the retail stores is Amazon.
Well, "any retail gift card" ends up being only a $25 denomination, because that's the only denomination cards that they offer. Someone else will have to help me with the math, but that would be either a 25% ($5 divided into $20) or a 20% ($5 divided into $25) discount. And, of course, using one of these cards would give you this discount on top of whatever other discount that you get on that ebook (or whatever it is) that you buy from Amazon.
Among the retail cards that Murphy offers is a $25 Visa gift card. It might be advantageous just to buy that card and you can use it for purchases at Amazon, Barnes & Noble, and a gazillion other places.
Murphy Oil's website has information about the promotion here:
http://www.murphyusa.com/Get-Deals/#!s2. Note that the promotion runs through the entire month of May.
FYI--Murphy Oil gas stations are often, if not always, affiliated with Walmart stores and located on their parking lots.