Are they giving you a cash discount or a quantity discount? Those are 2 different things. Quantity discount will save you more than a cash discount. They must love you if they only have to give you the cash discount.
Frankly, if you are buying that much you would be better off opening an account with the lumberyard and negotiating a discounted price for everything. Then depending on the business they might give you an additional 2% discount for payment within 10 days. Actually, if you are buying that amount of lumber, you would be better off not buying from a lumberyard but instead opening an account with a lumber distributor. Cash isn't king when it comes to business, accounts are.
Once again, if you are paying $10,000 with actual cash then they are reporting your payment to the IRS if they are following the law.