To be honest, I typically just count the price of things. When I used to shop by buying each part from a different reseller using Pricewatch, I never even figured in shipping.
I learned VERY quickly those many years ago that going to 50 resellers to save 2 bucks on each part didn't help...shipping usually killed me.
Even now I tend to order things from at most 2 or three places. I get all that I can from each place, even something that might be a tad pricey just to avoid another shipping fee elsewhere. I also try to keep places with decent shipping rates in mind.
Still, I never tend to count shipping until I get there. If you're doing a system (minus monitor) it's a safe bet to add up to 100 bucks for shipping, especially if you're getting 2-day. Thats probably as bad as it'll get, but still you'll wanna think between 50 and 100 bucks.
It's tough to do becuase shipping can change. Usually you have to fake build the orders and have shipping calculated. Then you can rethink things and buy later.
As for reselling, when I build things for friends or at work, I first price out the parts and then an estimate for shipping. Then when it's ok'ed I just do it, and give them the final price. If it's for a friend or relative, I will call them or show them the final price before execuitng the orders. (I don't charge build fees for family or friends)
For work, I just order things and let them figure it out later. I give them a ballpark figure, thats good enough. Sometimes I have to pay more for next day or second day shipping if its needed fast. It's understanded by them.
Also, when I do the final ordering, I somtimes find parts either out of stock or with prices changed. Then it's time to substitue...if you're careful you can get as good or better parts for only a few bucks more.