Sending mail overseas requires a bit more work than sending mail domestically. Even if you have the correct postage on your international mail, you can't just drop it in a mailbox unless it weighs 13 ounces or less. Anything larger than that must go to a United States Post Office retail counter or a USPS-approved shipper location. You can calculate mailing prices ahead of time on the USPS website by using its postal price calculator.
International mailing charges are priced by regional groups through the USPS. You can find which group your overseas mail destination falls into, along with rates, at the USPS website or at any post office. For example, first class international mail going to Europe, Australia or West Asia falls into groups 3-5. For an additional fee, you can send first class mail overseas -- other than postcards -- with options that include registered mail, return receipt or certificate of mailing. Not all countries permit return receipt or registered mail.
The USPS doesn't permit international mailings by individuals of alcohol, firearms, perfume or prescription drugs. Most perishable material is also on the prohibited list for international mailing. Check the USPS website for any restrictions on mailing items to your destination country under the laws of that nation.