Make sure you post it with on line tracking and signature on delivery. If it is an eBay sale, if you fill up all the info in listing correctly, it should show for her the postage cost. And never ever enter lower cost in the declaration, or write gift when it is sale. That is a criminal offense.
As far as I know there is no charge in US, but it is better if they make sure. Custom fees etc. will vary according to value; it is always up to the buyer to find out the info themselves, as paying it is their responsibility. Tell her to google it.
I am in Australia, and anything to value up to $1200 (postage included) is free; not many people buy anything anywhere close to that amount on line from o/s, therefore most people believe it is free for anything.