i have paid approximately 9% plus a fedex handling fee on a kelly. fedex billed me directly several weeks after receipt of the bag.
If the bag is shipped FEDEX/UPS/DHL/etc, you will definitely have to pay customs fees. This is because (unlike the post office) courier services are responsible for clearing the packages and these packages are never inspected by CBP (Customs and Border Patrol), other than random spot checks. failure on the part of a courier to pay customs duty on all shipments results in heavy fines and could potentially result in CBP revoking the couriers clearing privileges, which in turn would affect their delivery guarantees. This is not something courier services are willing to risk. The tax calculations and payments are built into their supply chain.
If the bag is shipped via the national mail carrier, which then transfers to the US Mail on this end, it's a bit of a crapshoot as to whether you will get charged customs duty or not. This is because the USPS does not clear the packages. Instead, packages are delivered to CBP and then released to the USPS by CBP after inspection. CBP does not look at every package, nor do they have the manpower or systems to levy duties on all packages above $800. Hence, lots of packages get through with no duty levied. Of course, delivery guarantees are not made in this case (just estimates) since things can get hung up in customs for a few days, perhaps even a week or two.