The program will stop when
variable is not "Y", and it has to be Upper case "Y". If you type "y" then the program will stop.
You can change
while (add == "Y")
while (add.ToUpper() == "Y")
About total multiple inputs
. You can make a new class that holds all information on all product added.
public class ProductInvoice
public bool Imported;
public string Product;
public float Price;
public float PriceWithSalesTax;
And then hold all info in a List<productinvoice>
] list and do call calculations there.