My experience is mostly with buying watches and shoes from overseas but a lot of it depends upon how your package is shipped.

If your stuff is being shipped UPS or Fedex, your chances of having to pay duty is much higher than if shipped via USPS.
In addition, UPS and Fedex will charge $25 broker fees plus some charge based on how much duty is assessed.

If you have to pay something, USPS typically won't give you your package until you pay the duty.
Fedex/UPS will typically deliver your package and then bill you a week or two later.