Hi Delta,
I'm in the EU and shipped various parts and watches to the US by registered mail and never had a problem.
If you look to insure the package I have no idea what happens, but without insurrance, there shouldn't be any problem.
Same the other way round with one difference: do not ship USPS boxes or enveloppes as they will be a tax for EU recipient with the risk the package is open by customs to apply further taxes (vat 21%). This of course can end as a seized pack if it contains a rep.
Use a plain Jiffy enveloppe both ways and no insurrance.
Cheers
Stephane