C# :: Materials Sold Bill Generation?

I've attached a pic of the program im trying to make, i'll explain in a few steps what it's supposed to do and what my problem is:

-User types in the amount of each material used.
-User click's the "Add" button, to add the materials to the bill.
-The bill is supposed to be shown like this:
material1 3x 1340
material2 2x 930
material3 7x 240
material4 1 840

Total 3350

-My problem is that i have like 100 different Materials, and how i should make the program only write the Materials used on the bill, instead of writing all the materials Down, even those which havent been used.

-every material has their own double variable, and a another double variable for the total Price of used material.

-right now it Works as shown on the Picture, i can add amount of each material, and calculate the total Price, and clear all the textboxes.