If you are sending packages/letters overseas to foreighn countrys,the United States Postal Service has an international rate calculator on their website to help you find out the aproximate cost before you go to the post office, online rate calculator

If you are using various classifieds around the net, Its always a good thing to be able to see if the trader is trustworthy,There is a very good auction site that has all its members listed with a feedback profile, Its worth a try finding out if your intended trader/buyer or seller is listed there, just use the search at the top of the page and type in the persons e-mail address, then click on the number next to the e-mail for a feeback profile. But the person your dealing with should be only too happy to offer such reasurance, a recognised good trader is loud and proud about his/her good status. ebay auction main page

Here is a link that has a lot of information to help make your internet experience a safe one where money is concerned.
The information is also helpfull if your currently trying to resolve a bad deal, or just want to know your rights. Internet police home page
OR.. The Scamwatch home page

The Internet Police also links to recognised good sites.

But nothing beats common sense where sending stuff is concerned, A few simple tasks could avoid a possble missunderstanding that could easily escalate into loss of trust and possibly a mail fraud acusation.
Always make sure you write the address clearly on your letters/boxes,
try to get proof of mailing out,
get insurance!.