Datagridview Probblem

Question

I m making an windows application form a restaurant managment system. i m searching the products in my billing section form using my database in a comboBox and the result will show in the datagridview. But the problem is that whenever i search a new product datagridview clear my last updated search and so that i cant make the total sum of prices of the products which user buy from me. Can anyone help .... 

Thanx in advance ....