I have not mailed any coins but the service works just fine for cards as long as your envelope looks just like any other mail going through the system.
I don't even use top loaders. I just sandwich the purchased card between 2 baseball cards, put that into a gaming sleeve (the kind people use to protect magic cards while playing the game), and tape that off-center inside the envelope. I alternate left to right from one order to the next so my resulting mail sits flat and isn't apparently lopsided.
I will go up to 5 gaming cards or 3 sports cards thick before putting a second sleeve of cards in on the other side. For larger orders I use cut up binder pages and spread the cards out between the 3 pockets. I have yet to ever have a post office issue with any larger order, every issue I have had has been with the smaller ones. In fact back in the first week when I was having trouble with them getting returned to my house the exact same clerk who I turned to for help examined both an envelope that actually had 21 cards in it, and another with 4, and she rejected the one with 4 and said the one with 21 in it was fine.
I don't use top loaders because they have more trouble getting through the system, the insurance always pays out anyway, and I would have spent $900 on top loaders last year and I had something like $30 in damage claims. Most of those claims I personally thought were fraudulent, but the insurance paid out on them anyway. The claims were almost all $1.99 Pokemon cards despite the fact that those aren't even 5 percent of my card business. Most of them came early in the service's history, likely from people who had previously been claiming not to have received their orders back when everyone shipped cards from stamps. I think I have only had 3 damage claims this year out of about 6000 envelopes.
I am getting zero dings for late shipments on standard envelopes. Looking at my late shipment rate report It shows about 10 standard envelope orders but every one of them says "Transaction Removed". Both the items on there are things that the post office sat on for a long period of time before they ever moved or got scanned. Even the stuff my postman scans right in front of me often never actually seems to show accepted until it gets to the hub.
