Sell at full retail for the moulding and matboard. If you can sell a 10' length of moulding that retails for $11/ft. for $110 and not have to make any cuts or measurements, that seems like an easy bit of change to me.
I don't simply sell to anyone that comes through the door but, after a few questions about what they are looking for and what the use will be, I usually tell them that the retail price is such and such and they go away. If they still want to buy a stick of moulding for retail price or a matboard for retail price, yeah, I'll sell the merchandise. That is money in my pocket and they aren't going to put a serious dent in my business by ruining a couple of matboards or a length of moulding trying to save a few bucks over having me do the job correctly.
And, in the end, they may well come back to have it done in my shop when they find out that it isn't as easy as it looks.
Framerguy