I have started to make plastic parts, I dont have an account, I havent as yet ordered more than 5 sheets of plastic at a time. I dont think you can go there and just buy stuff on the spot or at least that is what the helper monkeys implied ( it was never answered directly).
What I do is order online and pick up at the will call door is easy and they guys there are Extremely nice and helpful. They even cut some sheets for me at no cost on the spot. They will be getting more of my money.
To pick up at the warehouse you need to make sure they have it in stock which I always do thru email, which they respond to quickly. Then you mark freight collect when checking out using the account 0000000 and I alway put in the comment pick up phoenix. It works for me but if in doubt give them a call