Summa Theologica, Q78a2 reply to objection 7:
If a man wish to sell his goods at a higher price than that which is just, so that he may wait for the buyer to pay, it is manifestly a case of usury: because this waiting for the payment of the price has the character of a loan, so that whatever he demands beyond the just price in consideration of this delay, is like a price for a loan, which pertains to usury. On like manner if a buyer wishes to buy goods at a lower price than what is just, for the reason that he pays for the goods before they can be delivered, it is a sin of usury; because again this anticipated payment of money has the character of a loan, the price of which is the rebate on the just price of the goods sold. On the other hand if a man wishes to allow a rebate on the just price in order that he may have his money sooner, he is not guilty of the sin of usury.
So is “90 days same as cash” usury on this principle? Or is it just the mirror image of Thomas’ last example, a merchant “allowing a rebate on the just price in order that he may have his money sooner”?
On the one hand, it appears that unless usury were common, offering a zero-interest loan would have no advantageous effect for the seller. It is in effect a discount to undersell competitors who cannot afford to offer such a loan.
On the other hand, there is no immediate disadvantage to the buyer, as is usual in cases of usury.
Of course, the probably expectation of the seller is that some of the buyers will not pay before the end of the zero-interest grace period, and will therefore be charged interest after all. The zero-interest loan thus falls afoul of Aquinas’ other reasonings eventually.