Please DO NOT use the US postal service or any national postal services. They can take up to 2~3 weeks and that is way too long for any school to wait for the documents.
Please use FedEx, DHL, UPS and other private express mail carriers that you can get a tracking number from.
This can mean the difference in landing a postion or losing a position you already had.

* If you do not have a choice and you must use the national postal service, then  ask for EMS(express mail service). This is the fastest mailing system that a national postal service has. I believe all countries have this system or similar to it.

What ever system you use, use a service that will not take over three days.