I believe that is how it works.
You tell them which store you prefer, and then list the items you wish them to buy - be as specific as possible concerning brands, sizes, quantity, etc.
They ask on the Website if your item is not available, if you will accept substitutions.
On a recent thread, there was a list of stores, Super Wal-Mart, Publix, etc.
I plan to "shop" at my local Super W-M before we leave to check on brands, sizes, etc. Otherwise, I will accept the best substitution available.