“I’ve been filling the mailer with old metal washers that I get from a friend who repairs cars. He gives me all of the old washers and I store them until I need to “respond” to AARP mailings. It’s very satisfying ... “
Your washers are probably not making it through the mail sorting machines. They use pinch rollers. A friend sent me some shim washers for an old Shobud I was restoring. There was a hole in the end of the envelope where the pinch rollers had pushed the washers right through.
You need something flat, and about the same size as the return envelope. Might I suggest sheet lead, used by roofers as flashing. Easy to cut with heavy duty sheers, and pretty cheap from Lowe’s or Home Depot. And HEAVY!
Thanks for the updated idea ... that sounds a lot better than the washers. Now that I think about it, I may have some of that in my shed that was left over from the last roof that I put on my room extension.