Hi, I've done quite a few custom orders, in fact, I did one last week! Agreed, the initial interface is extremely basic, and estimating shipping costs without the customer's address is less than ideal. The thing that has always worked best for me, is to update the listing right after it's created and then add any listing photos. It normally notifies the customer in the message window after each update.
In fact, there's almost always an error that the shipping information is incomplete, so you'll have to go back and set the carrier and processing time. Perhaps you can use table rate shipping here too... never tried it? As another seller suggested, I've found it's less of a hassle to create a public listing and earmark it for that particular customer with their name in the title. That way the shipping cost would reflect the customer's location. Hope that helps!
Another thing to look at for, once the customer purchases the item, you will not be able to edit the listing or update the photos and you've established a contract.
