You have my sympathy on this on.   Years ago, long before I ever started with USPS we had a carrier that did the same thing.  For years we had grab every catalogue and magazine and hold it by a corner and shake it to see what fell out.   I know a lot of people accidentally threw out a lot of mail not knowing it was tucked inside.    I don't really have an answer.   You can try leaving him a note or talking to the postmaster but it's tough to get carriers to change their system.    As we say at our office, you can't teach an old carrier a new trick.

I've never heard of a carrier that rubber banded the letters together, seems like an unnecessary effort.