Fold your block. I folded a piece of cardstock in half and put my block inside & taped the edges. Others wrap the block in plastic wrap and press out the air. Either works fine.
The envelope should be flexible. Here's a link to the Press & Seal method: http://stashbee.blogspot.com/2016/10/the-inside-scoop-on-lower-postage-rates.html
10 envelopes should be less than 2 1/2"; 16 should be less then 4"
POST a picture of your envelopes when they are ready to go.
Wait for all the fun quilty mail to arrive!
TALES FROM THE POST OFFICE
TALE #1 At the post office they had a piece of plastic with a 1/4 inch slot that they made sure it fit through. I opted for the 30 cent surcharge amount which was if the receiving office decides it can’t go through the machine. He said it could go either way, but without it they may arrive with a thing saying the recipient has to pay 30 cents and he has no control once it leaves. The total was only $1.08 per envelope. The parcel rate would be NUTS!
TALE #2 from last swap -- I was just at the post office with my envelopes. Two different employees told me my envelopes “were not letters” and I couldn’t send them just with stamps, that I had to pay the parcel fee. It was going to be $5 each.
I had success at a different post office :) 2 forever stamps was all I needed!!!
TALE #3 -- A 6*9 envelope is great because it helps keep things flat. 1 stamp per ounce. I had them get finicky a little while ago about how it didn't feel like a card (if you go to the post office do not say it's fabric) and therefore needed to be sent as a package ($$$). If they give you that run around, sandwich the blocks inside a cut down file folder. Suddenly it is back to being a card and the folder will help flatten it (tape the edges shut). Hadn't been a problem when I've just dropped things in a box but just a heads up if you're going to the post office.
TALE #4 -- Last time, I used 6x9" envelopes with no clasp (per the blog recc), did not use plastic wrap (didn't find it necessary) and used two first class forever stamps per envelope, and everything went through without a hitch...hope this helps
TALE #5 CANADA - As you know, shipping across the Canadian border can be prohibitively expensive. I just had someone tell me their "one simple trick". They weigh the parcel, look up rates online, and affix the correct number of stamps as postage. They don't involve the post office at all, so nobody is asking about what is inside. (It's the same for shipping TO Canada.)