Some offer free seeds provided you pay for postage, i.e., no trade needed. I assume your postman picks up your out-going mail from your mailbox, or there's a post office or post box walking distance for you to mail out reimbursement of these "SASEs". If you are planning a larger trade, you might have to go to the post office. Although I live close to a post office, I usually do my own weighing and online postage for priority mail, then just drop off at the post office. They do provided delivery of free, priority boxes to your home, but I just checked and see they only offer the Priority Mail/Express and the Flat Rate boxes. Don't know if I've answered your question.

Thanks. I did not know about online postage.

After joining/signing up to USPS, you can order diff size Priority boxes FREE. That way, you can see what size you need; The smallest box is $7.15 and UP.

If you know when you will ship the Priority box and they even pick up from your mail box. ( Just make sure with your mail man)