I use Davidson's also. I work on a cash basis, ACH debit per sale. They, as other distributors, do not restrict you to using just them. If they, or anyone else, did, then you'd be pretty much screwed when you go to order and they don't have what you need but someone else does.
My biggest problem with Davidson's is that they haven't had what I needed to order when I needed it. As to cost, I find them a bit higher often then my cheapest distributors (not always), but by far not as expensive as Lipsey's. They by far are my most costly wholesaler.