The best way to use drop shippers is either set up a (set price amount) with each of your companies, so you can already know the shipping chargers obvioiusly. Or you can also just get a general amount for each drop shippers charges and use an average, you will lose some money on occasion, but it will be trial and error for a while.
The best way is to buy or build a program that will add your drop shipping charges for you. You will be able to add multiple drop shipper and their chargers and it will do all the work. If you would like info on a good "warehouse" program let me know and I will give you a bit of info you can look into.
Sam
VP - www.tradestuffup.com