Ok i have sent lots of letters here's what i got.
1. you don't need bubble envelope
Use a standard letter and put some kind of padding. I use dish(like china) foam used by moving people. 1/16in
here's a link to see what im talking about
2. Put it in a normal letter envelope
if you want people to charge you letter rate... use a letter envelope.
3. Secure the contents, must be less than or equal to 1/4 inch thick! if you go over its a package!
Use some tape to keep everything spread out. If you don't and all the seeds collect into one corner of the envelope it could end up being too thick and you will get postage due.
4. Use for only a few seeds use 1 stamp, for 50+ seeds 2 forever stamps, international use 3 stamps
1 forever stamp is because the envelope is normal
2 forever stamps is in case they mark your envelope unmachinable aka its lumpy
3 forever stamps because international letter rate is 1.20$
if envellope is over an ounce, will probably need to take it to usps office.... or just separate into multiple letters.