You do not need a mod

The feature exists in Zen Cart. Quantity discounts. Go to

admin>>catalog>>product price manager

select the product then go right down to the bottom of the page and click edit. then click 'add 5 blank discounts' then you can add up to 5 levels of discounts.

gets a bit boring for lots of products. swguy has written a mod that applies quantity discounts to products, categories or the whole shop. I have a discount copier out there that copies discounts (who'd of thunk it) from one product to another or to a whole category...