Lightest, secure and inexpensive way I have seen to package and ship seeds is place your seeds in Graduated Microcentrifuge Tubes (Cheap on Amazon) and label with a piece of tape. Then tape the tubes to a small piece of cardboard or similar. Then just fold over the cardboard or similar and slap in an envelope/package with 2 stamps. The cardboard might be a tad thick but it definitely gives the seeds a secure ride through the mail. Just my 1.5 cents