Author ~ DizzyDaffodil 

SASBE is a Self Addressed Stamped BUBBLE Envelope -which may be a bit confusing for some as it is really just a shorter way of saying "postage, address label, inside a bubble mailer".

The easiest way to send a SASBE is to send your postage, clearly written address label and a note
 introducing yourself with user id & real name to remind the recipient who you are & what you wanted - all inside the actual
 bubble mailer you want them to send back to you.
All they have to do is slap on the label & postage & fill with goodies. 
  Most traders reuse bubble mailers to death so this will not be a surprise for them at all. 
You do not have to send the bubble mailer inside another envelope.
If a bubble is requested, you should send a bubble mailer, not a regular envelope as many seeds can be damaged in the mail if not treated well. Keep in mind, postage is higher for anything thicker than 1/4 inch and with more weight. You can calculate postage costs at the USPS web site for free. Hand canceling is 17 cents extra. 
As a person who has received numerous goodies from generous souls: 
If I am not sure how much postage will be required, I always include extra - I figure this person is being generous enough to take their time and items/seeds to share with me, the least I can do is be thoughtful enough to ensure it does not cost them anything else. I've found most traders will return any unused postage with the seeds, if they don't it still cost me less than the seeds would commercially. 
When you get your package, if you see it cost more than you sent, it would be nice of you to return it or at least offer. 

How many seeds should I expect? If you are concerned with this, you need to ask up front before you ever send postage. Usually, I would not expect any more than I would receive in a commercial package (small amount) and am pleased to get that, as this is free to me. Some things have been shared with me in incredibly generous amounts, and some things that are harder to get, only a few seeds. 
 

Trader: How many seeds should I send? Send as much as you like - If this is something common that you have a great deal of, send more. If it is something rare or harder to find, send less or a few - Just make sure you are clear in your communications that you only have a small amount to share. 

What is it? Some traders don't take time to put in a note reminding ME what they are sending, so I try to make sure I put something in an inconspicious spot on the envelope to remind me what I am getting. As an example: put MG in the bottom corner if I am looking to receive morning glory seeds back in that envelope. 

Got it - now what?
It is always appropriate to publicly post your receipt of the items and to thank the donor. 

As a trader: I have spent a bit of extra
postage myself at times, ensuring that my seeds get to someone safely as they weren't sure how much to send and didn't know to ask about getting seeds for postage. I don't mind if I think it is an honest mistake or a newbie, but many folks can not afford to be out extra postage.  
I see that some of this becomes a confusing issue at times, so thought a "Best Practices" might be helpful for both sides. 
Whatever your issues or problems, remember to be courteous to others. No one wants to trade with a grinch or a grouch.

DizzyDaffodil