Exactly why is it not ribit to charge disproportionately more for a single item than for each unit in a bulk purchase? Of course, I'm talking about Jews' selling to each other.
People have requested an explanation of why I think this is ribis. I think it should be unnecessary to explain, because people could get to the explanation just by thinking about it. But, in the name of taking some of the burden off of readers and potential answerers, I offer this discussion:
@DoubleAA points out that it is ribis to deposit money into a Jewish-owned bank. This is presumably because the bank (a Jew) pays you interest for the privilege of keeping your money. By the same token, it seems it should be ribis to place your money in sequestration with a merchandise seller who pays you interest (extra goods [not just more goods]) for the privilege of taking all your money up front.
Or perhaps the reverse is true. Could it be ribis on the seller's part to charge you interest, as it were, for keeping your own money (i.e., when you buy a smaller size for a higher unit price)? In other words, in the case where you are buying individual units one at a time as needed, you are taking longer to pay the seller for the same amount of merchandise, and you are paying interest for it in the form of a higher price for that merchandise.