Why can't you provide the shipping costs without having to check and get back to the customer ? You know their address and you know the address of where the parts are being shipped from.
Tell us the reason for that without trying to make yourself look better by comparing to other companies. Are customers not more apt to accept your shipping charges the more they get into making an order ? Some may only need a little time and some may need a considerable amount of time to find their items and order. Your policy seems advantageous to you and a disadvantage to customers.