food delivery site - offers rules

hi ,I' m developing a online food delivery site.
All fine but now I want to allow the restaurants to apply offers rules
. i.e. if you order 2 pizzas you get 1coke for free or if you order for more than 100euros you get 10% discount or something else.
It must be flexible since different restaurants has different offers.
Any suggestions how I should approach this issue?
 
I store all the products in an sql database.
Thanks,
Stelios
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