merging sales order several materials into one material in invoice

if I have 2 products of same price that I am selling in one order but I need them to appear as one material in the invoice to the customer, how can this be done ?

in SO I have:

200 Tons of material A with a price of 50 USD/Ton

150 Tons of material B with a price of 50 USD/Ton

in customer invoice I need to see:

material C - Qty=350 Ton - price 17500 USD