Wal-Mart does not charge shelf fees. Instead, they want to stock the most popular products (the ones they can move the most of) and buy in a big enough volume that they can exert pricing pressure on the operations selling to them to get a better deal.
Say you make a great new motor oil, you pitch it to Wal-Mart and you are advertising like crazy. If the demand is there and Wal-Mart feels like they could make money selling it, they will buy your product. However, you have to deliver on time all the time, you have to meet their logistics standards, if you do not change your product be expected to reduce your wholesale cost on it 5% annually, and they may make other requests.
For example, if you sell to everyone else in gallon jugs and the retail price is $16.99, Wal-Mart may want you to produce the same product in five quart jugs and they feel that if they buy enough volume, you can sell them the five quart jugs for the same price everyone else buys gallons, and they will sell those five quart jugs for the same $16.99 everyone else is selling gallons for.
They didn't get big by being stupid. As evil as Wal-Mart is accused of being, if you hold up your end of the deal the cheques are always on time and they will hold up their end of the deal.