I always ask for USPS (US Postal Service) shipping it is far cheaper than the courier firms like UPS, FedEx etc.
I have paid up to $140 for a complete bike.
The seller should break it down and it should then fit into a carton that meets with the size requirements for USPS.
You can get a tracking number that will work both on the USPS and UK post office/parcel force web sites.
Ask the seller to put this code number 9501009000 (this is the tariff code for bicycle wheels and parts) when they send the bike, it should be clearly stated on the address cover letter on the package.
This should legitimately reduce the import duty to a zero % rate, but as kdw says you may still have to pay VAT and an admin charge.
All above for a 20" only, I have not bought a cruiser ... yet!
Regards
Paul