Hello. I have a few tips. I'm super-new at this, but so far I've found a few ways to mail ACEOs without incident. I took old audio cassette tape cases and broke off the 2 "prongs" inside and stuck my business card over that ('cuz it may be sharp) with double-sided tape. Then I put double-sided tape on the back of my ACEO (canvas) and stick it on the other side inside the case. I don't like putting tape on it; but since I'm using canvas and painting with oils, it works out really well for a temporary housing during shipping. I've also used this same method but with CD cases that are emptied of the CD-holding layer. I don't want anything to rub against my painting during shipping, so this has worked great so far. (I've even gotten compliments from buyers.) And, the plastic cases are super-strong compared to many other options. (Note: CD cases do flex a bit, so be sure to place your canvas nearest to the opening edge, not the "hinge" edge.) The last few steps: I ziploc baggy it and put it into a bubble envelope. Then I tape it up like crazy, and also put a layer of clear tape over the person's address - so in case of water or other hazards, the address will remain intact. (My mother sends care packages to Russia, so I am well-trained in the "Wrap it like it's going to Russia" method.) 🙂 I will continue using this method until I can find a better one. I know I will not use this method when I send watercolors, since the double-sided tape will not be a good option for paper. I'll keep brainstorming and I'll let Groups know if I find a really great way that's not too costly.
Good luck to you!!!
-Jendria