"Stretching" Accounts Payables (i.e. paying accounts payables past the due date without remitting the penalty, or paying after the discount period has expired but still taking the discount) is a common way that some firms gain cheap financing. However, a firm can only get away with it if they have an imbalance of power between them and the vendor. Here are a few Dilbert cartoons that illustrate the basic idea almost perfectly:
Unfortunately, unless you're a Wal-Mart (notorious for abusing their vendors), the flyswatter eventually ends up in the vendors' hands. So, "free" isn't always "free".
If a textbook vendor was smart, they'd just use Scott Adams for most of their illustrations.