Add cost in oscommerce-based shopping cart when option field is true.

For a webshop running a modified oscommerce, I need a change in the shoppingcart. Whenever a product with the products_opt3 (in the products table) set to "true" is added to the shopping cart, 350 grams of weight needs to be added to the cart. The extra weight can only be added once, even if more than 1 product has the option field set to true. For example: 10 articles of 100 grams and 5 of them have the products_opt3 set to true= a total cart weight of 1,350 kg