One reason those types of stores are able to usually sell a little cheaper is because they refuse to hire knowledgeable people in the departments that require it.
This is especially true in the tile aisles. So much wrong info dispensed there. Shop there at your own risk is what the sign should say.
You do not wait 28 days for the deck mud to fully cure, just like you do not have to wait 28 days to park on your new driveway. In the old days, the recommendation was to set the tiles in the "still plastic mud" or wait 3 days for it to cure. It depended on the size of the area to choose which.
You can install your tiles the next day, overnight drying is enough. Or, when building an entire shower, you can cover the deck mud with cardboard and set the tiles near the end of the project.
The proper mix to use is "Sand Mix", some call it "Sand Topping Mix" http://www.quikrete.com/ProductLines/SandToppingMix.asp
It comes blended in the bag at a 3:1 ratio, ideally a 4:1 or 5:1 is used for floors. The 3:1 is great, but a bit harder to work with, so you can blend some more sand to it if you like. The ratios are shown by volume not weight.