50c for padded bags in bulk? Mine cost 12c by buying in bulk! That was from the same seller that the OP left the link for and the item number I left further up. I was originally paying about 50c from Officeworks, which is when I looked into the buy in bulk option.

I couldn't get my PO to part with a letter gauge, that's why I mentioned that I'd made my own from thick cardboard. It works great and have never had any issues with items being too thick. I think I may have even made the slot 19mm just to make sure it didn't go over.