Looking for a way/formula to calculate a number based on a selection of specific products

Hello,

We are working on a pretty odd way to give discount to our client based on wich products they purchase combined to each other.

I would like to make a verry  simple excel using checkboxes where you select the products and you are given the percentage discount you receive.

The discount chart is as followed:

A + C + G = -5%

B + E + F + I = -5%

D + H + J + K = -5%

A + C + G + E = -7.5%

B + E + F + I + A = -7.5%

D + H + J + K + C= -7.5%

A + C + G + B + E + F + I = -12.5%

A + C + G + D + H + J + K = -12.5%

B + E + F + I + D + H + J + K = -12.5%

ALL PRODUCTS = -20%

Thanks in advance!