calculating postage costs based on weight but only if postage is required.

hi

i need my spreadsheet to be able to calculate the price to add on for shipping based on how much the order weighs.
however, sometimes an order might not be posted and so no shipping costs will be required.

the postal rates are calculated using the formula =IF(SUM(C12:C21)<=50,12,25)
the weight of each product goes in c12:c21 and then if the total weight of the order is less than or equal to 50kg, it will add on £12 in shipping costs, otherwise if it is over 50kg in weight it will add on £25 in shipping costs.
the problem i need to solve is sometimes postage is not required to be added onto the order because the customer is going to be picking up the products.

i had created a cell where the user can type in a "y" or an "n" if postage is or isn't required but i cannot construct a formula that will then either remove the postal costs or add the costs in based on the weight as outlined above.

i've attached a screen grab of my spreadsheet to help give you an idea of the layout.

how would i implement the solution i've mentioned and what would the formula be?

thanks for any help.

screengrab.png

I don't see the cell where you have the "y" / "n". Assuming it is in A1, the formula would be

=IF(A1="y",IF(SUM(C12:C21)<=50,12,25),0)
Does that work for you?