I am not the best; but this is what I do
I goto USPS official site and follow the directions and print up the customs forms-then almost invariably when I take the package into the local post office it is always $2-3 dollars cheaper than the USPS web site.....
Oh yeah I weight the boxes up on a small digital scale.
I kind of wish there were a more realiable site.
however I have a good idea what the costs should be now....because all my shopping except a quarterly trip to wallmart and groceries is done on line.
I don't know much about shipping internationally either. It seems that each country has different rules. I'm fortunate in that I live in a very small community so I know the folks at my post office and can go in and ask a bizillion questions and not have a huge line waiting behind me. I do believe there is a packet you can get from USPS about international shipping. I've never had much luck with those digital scales...even for domestic shipping...seems like the postage is always different by the time I get to the post office anyway so I don't even try anymore.
It's really not that hard...just more paperwork. I think the other end has more hassle then we do with duty fees, customs, etc.