Minimum quantity with different combinations

[Kirby]

I've been searching for days with no luck, let's see if someone can help me here.

I've almost finished a new Prestashop  on 1.6.1.5 for a t-shirt company. I've got a special category where you can buy some products at lower prices, with the condition of purchasing more than 15 units. 

What I need: People are supposed to be able to buy, for example, 5 size S model A shirts, 5 size M model B, 2 size M model C and 3 size L model C. They HAVE TO  buy AT LEAST 15 units, no matter which models or sizes.

What I've done: Made a "special offers" category, made a "shirts" product inside, with combinations both for models and sizes. Set the minimum quantity of each combination to 15.

What I've got: People can add a minimum of 15 units of each combination, but can't add X units of each until they've got a total amount of 15.

I've been looking for some modules to do this, and found some of them that looked very close to what I want, specially some from Vekia. I've already tested some demos out there, but I can't find this functionality anywhere, which I think is a pretty common need. I need to find and buy this module ASAP since it's practically the very last thing I need to finish the work.

Any ideas?