As Fekin says - build a relationship with someone in the store.
(Get yourself a "champion" to use a sales phrase.)
It's also well worth having an open - straight to the point and friendly chat with the manager and staff about just how much info they are giving to the customer about your side of things - i.e. prices / grout joints / etc.
Early on I found that the staff were quoting all kinds of weird and wonderful prices - ranging from the dirt cheap to the ridiculous! I simply asked that they don't discuss price or fixing techniques AT ALL but leave it all to me.
I assured them that I would "sell" the service and educate the customer as to why they needed certain things and how I would do things. I think the staff were actually pleased to hear this - sometimes they must feel under pressure when customer fire questions they can't really answer.
Just ask them to leave this side to you - it worked for me!
(love that quote about the customer wanting monolithic look marble!)
Also - make sure you tighten up on your quotes to include specification like grout joint widths etc - get a signature before you begin.
