Part of the high shipping cost was DHL doing all the export/import paperwork. Could be Czech authorities have looked closer at their regs or misinterpreted them. Or USA authorities think the word "Czech" is part of China and hence Chinese guns, parts and ammo can't be imported.
Pretty dumb though as CZ sends complete guns and parts to us. Maybe CZ-USA complained. Lots of could be's and maybe's. They probably won't say for sure.