Like WCC said, shipping overseas with USPS is just about the same as shipping a package to anyone within the US. The only difference is the customs declaration sticker (CN22 form) that has to be on the box.
http://about.usps.com/forms/ps2976.pdf
You can this sticker for free at any post office and you only have to fill out the following information:
- Tick box: Gift or Merchandise. (makes no real difference, only the declared value determines if the recipient has to pay import duties)
- Detailed description: (knife / pocket knife etc...)
- Weight of the item
- Item value (the stated value will determine if the package has to go through the customs process in the destination country or passes without further processing)*
- Sender and recipient address
You do not have to worry if the item is legal to own / import in the recipient country, That is the responsibility of the recipient.
If it is legal to own / ship in the US than it is legal for you to ship abroad.
If the item is illegal in the destination country than it might be confiscated by customs but that is not your problem. If you worry that the buyer might try to file a Paypal item not received claim, than ask them to make the payment as a non-refundable "Send money to friends and family" payment.
I usually opt for First-Class International which is the cheapest option and the ETA is usually about 2 weeks.
Priority Mail International Small Flat Rate Box is a good second option and a bit faster, ETA is usually 1 week.
Here is a list of available options based on a package with a weight of 1 lbs:
http://ircalc.usps.gov/MailServices.aspx?country=10086&m=6&p=1&o=0&dpb=0&mdt=2015/09/17%2008:00&dvi=20
*About the declared value......
Officially you are required to state the actual value of the item but this value is only of importance for the customs office in the destination country as based upon the stated value, they will decide if the recipient has to pay import duties or not. If customs in the receiving country has doubts about the declared value, they might open the package to inspect the contents and to see if there is an invoice included but usually they will accept the declared value. Worst case scenario if they really can not believe that the stated value is correct, they will just make an estimate or contact the recipient and ask them to provide proof of the value.
If you worry about any legal consequences for not stating the actual value of the item if the buyer asks you to declare a lower value, than note that this concerns the customs laws of the destination country, it is the buyers responsibility to provide the correct information, not the sellers as in that case it is the buyer that breaks the
import law in his country, not you.