Digging through the forums, I've found that this has come up in the past (2009) but hasn't seemed to resurface lately. Site owner needs the ability to mix & match products within specific categories. For example, if a customer buys 10 of 1 item, 20 of another, and 3 of another, they get the price break for 30+ items on all 3 products, even if it is a different price.
Another way it was explained:
I've found a couple of PHP carts that can do it, but I'm just not exactly sure how they are doing it.A retail customer orders a quantity of 3 cheap product Y at a price of $2.25 each. They also order a quantity of 2 expensive product XX at $34 each. (Individually, if they order just the 5 expensive products, the price goes down to $29 each for a total savings of $25 for the 5 expensive products.) This customer has 3 cheapies, and 2 expensives in their cart that would trip the volume pricing of 5 for the expensive product and give them a $10 total discount which is *more* than the cost of the 3 cheapie products.
http://www.beartoothwoods.com/catalog/mix_match.php - looks like they may be doing it by category
Has anyone come up with something that does this? I'd hope so by now, since the question has actually been brought up all the way back to 2003 it looked like when I searched.