Mandarin is correct, they are easy to hook up (Dare I say absurdly easy?).
-- Water source: This can be the kitchen faucet (And some units come with the appropriate adaptor), as mentioned the water source for a washing machine with the Y-fitting (Or even gardenhose tap), or you can physically tap into a water pipe. The AirWaterIce unit I got came with all these fittings.
-- Drain/waste-water: If in the laundry area, you can use the drain for the washing machine. Mine was merely an open upright pipe, so I just stuck the waste-water tube down there. I imagine most units come with (like the A-W-I), or offer the means to permanently tap into a drainpipe (Most often seen with under-the-sink installations). If set up on your kitchen counter, the tube can simply be put down the drain.
The A-W-I unt I have utilizes those simply slip-joint fitting for the small tubes, no tools necessary in that regard (other than scissors to cut the tubing). For a permanent installation, the unit comes with a self-tapping fitting for source water, and a harness requiring drilling a hole for the waste-water drain.
Don't be discouraged by a potentialy difficult installation, these things are a breeze. Mine took a little over an hour to put in, and that time included a trip to the big-box homestore to get a fitting since I ordered the wrong one when I bought the unit.
Frankly, I haven't had to deal with non-RO/DI water, or store-bought, but I can't imagine being even semi-seriously into this hobby and not having RO/DI water in-house. There are just too many advantages.
BTW, good on your LFS for dealing with the problem. It's easy to be a good business when everything goes right, but the true test is when something does wrong . . . do they blow it off or do something about it? Sounds like this store at least knows when a mistake is made and is willing to deal with it.