You don't have to prepay customs with the post office - the receipient is responsible for it. If you use an on-line service like Stamps.com or Paypal shipping, you get a bit of a discount on the mailing cost. All merchandise is required to be sent as a package - no envelopes or large envelopes - so you are automatically in the $14+ range for postage.

Services like UPS and FedEx add "brokerage" services where they pave the way through customs, but for occasional users, merchandise that doesn't require export licenses, and small transactions this is an unnecessary and extreme expense