Revell 1/125 early war VII type. (A VIIC I think) There is a resin conning tower available from Nautilus and I believe PE as well. Nice display size.
Ex-Gulfstream, ex-BWN, now YKM 1/350 U-boats. Resin with PE. Both early war and late war types.-very small kit.
Mirage 1/400. Many available with PE+ U-boat pen diorama available. -very small kit.
Avoid Asian 1/150 U-boat. Usually kitted as U-581. Available from Academy and others. Designed for motorized and porpoising action. Single propeller.
Aurora/Monogram 1/209 U-505 model which is an IXC type. Very old school. Can use generic 1/200 PE to dress up the kit. This kit is still available at the U-505 museum for about $13/$14. A bit on the smallish side for lone display.
I think just about any assembled and detailed model will likely suffer damage during transit.
Here is a nice 1/125
http://www.modelshipwrights.com/modules.php?op=modload&name=SquawkBox&file=index&req=viewtopic&topic_id=67370&page=1