USPS will even ship packs of the flat-rate, padded envelopes to your house for free.
https://store.usps.com/store/b...
I've even had someone mail a pair of shoes that I purchased in them (although I wasn't too happy about that.)
They currently cost $6.10 to ship. You'll want to check that the post office doesn't have a scheduled price hike before the end of your swap.
If it were me, I do like Ella and round it up to $7 to cover costs of any extras that I might need to use like packing tape, printer ink, etc. (The envelopes have their own seal, but really stuffed packs may need extra.)
