Both. If I buy a dozen 2x4x104 5/8" pre cut studs, I don't get a volume discount but they give me a better price for cash. The quantity discounts don't usually apply to small orders.
Paying cash is a discounted price. Cash is king when you can pay the bill, get the same price as what you're saying with an account, and you don't have to concern yourself with having to keep up with paying later.
Do you consider a check the same as actual $100 bills? The money is there to back it up when it's written and not having it would mean they could report me to the police for knowingly writing bad checks.
So, you aren't using cash. OK, got that one. It seems your claims of "pulling out cash" were nonsense just as I suspected.
The thing is if you have an account at a normal lumberyard they give you quantity discounts for everything because your yearly volume earns that discount. Your account rep looks at it and says, he buys $10,000 worth of studs every year so our standard price for him is $6.40 per stud vs the $7.00 normal price. They then charge you $6.40 per stud if you buy one or if you buy 100. Then they would bill your account and send you a monthly invoice. Many businesses like lumberyards then offer a further 2% reduction if you pay within 10 days instead of the 30 days.
Having money there to back up a check is no different than having money there to back up a credit card. The only difference is when you use the card you get cash back.