I think if it was only one or two it would be OK, but I dropped 8 large letters in the box today! That's unusual for me though, usually I post one or 2. Thankfully, my little PO rarely has a line, so I could probably get away with it, but at larger, busier ones it could be an issue. 

I guess one solution would be to get them to stamp them, then move aside while you photograph them, allowing the next person to be served.