The way I do it is to bid on the item and send him a message when it ends to ask about shipping cost to the US. He adjusts the total accordingly and sends me the final bill.
I'm guessing here, but maybe he's ignoring your messages because he has never dealt with you before?

Most of his line of Panzer IV upgrades are listed as of now on his fleabay shop page, just go in there and bid on what you want (pm me for a link to the page if you don't have it already, it is a pain to find via the search function - if you don't see 20+ items listed you are not on the right page!). Send him a PM after you have bid asking how much the shipping is and he will tell you (it will be around 12 euro's).

if you have google toolbar installed there is the right click option 'page info > translate page into english' otherwise you are looking for a link called "E-Mail an Verkäufer senden" on the item's page

best of luck!
Martin