VirtueMart

I have code in place in VM1.1.x system for this functionality (similar I guess) and I can send it across if anyone wants.
If gives the user configuration to enable / disable this system and choose what product should be added if total exceeds a certain amount.

Also in-place is an option to make the lowest value product free in 1 quantity if the number of items in cart exceeds a certain number (configurable).

ahh also great. Then please take a look on it. But it does not help in the old stlye. Polisheed geek already wrote it in MVC and wanted to add it. But another input could help. There exist different ideas for this system.
Some want just give a rabbat on the amount. Others have the wares in packs and want to give a discount, when they dont have to break a package, like Buy 6 get 5% procent, buy 12 get 10% and so on.

The system can already hold the product (should be id), the unit (kg, g, per unit, ...) and the kind of the discount. The system makes it possible to have multiple discounts. So some of this ideas can be already almost done with different rules (for exampel for every number you meet a package size you add another rule)

Others have this system buy 5 get one for free. The rulebase system should make it easy to add all this features. with a simpel include we could make it possible that the people add their own customized rules.