add price
If I’m buying stuff and tell the seller that I will add 0.5$ in their product, is my sentence makes sense?
The seller gave me a quotation before. I’ll buy some of the products form the quotation list, the prices are basic form the list and add 0.5$.
Each carton need to attach a date label so we add 0.5$ in every carton to cover your additional cost.

Question

Answer

Not really. You might say something like
If you add a date label to every carton, we will pay you an additional $0.50 per carton.
Would you be willing to add a date label to every carton for an additional $0.50 per carton?
We can pay $0.50 above the quoted unit price if you add a date label.