Most homeowners can get better pricing if they just call first and see what price is offered. If your coming in for a single part sure maybe you might pay a bit more but if your picking up parts for an entire bathroom you can most times get a better rate.
The box stores are like 7-11 stores and Mac stores. You can buy milk there but it's always cheaper to go the the grocery store.
Homedepot and Rona sell Kerdi for $3.00 a square foot. one 2"x4" might be cheaper. Maybe a length of pipe is cheaper. But then you get hosed on the fittings, screws and the like.
Just say no to the box. Just say no.