Wich plugin for discount

I got the plugin whole sale suit

and i have it working decent at the moment

Problem is i can apply a discount per item in the shopping basket.

Problem is

i sell different kind of meads (Honey Wine) in different size of bottles

so i would like to offer my wholesale clients to get a discount on the amount they buy

so buying 100 1 liter Botles gets a better discount than buying

100 0,5 Liter Botles.

is there any plugin that takes care of that part i was thinking about giving it custom attribute saying liter = 0,5 etc

Peter

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