Normally its worked off of Value, might be low enough that it'll just be the standard fees set by USPS as I didn't have to pay any extra duty on parts and ammo I was sent.
Shipping will probably be between 10-20 quid but don't quote me on that as its been a little while since my last import, if its fairly light it won't be that bad.
Time wise it may get stopped for a check so allow an extra couple of weeks if you've got a time you need it for, they will know how to do the paperwork otherwise to ensure it gets through.