Hence the "friendly reminder" and warning that if they don't explain or remedy their lack of payment you will charge interest like everyone else. You are willing to give them the benefit of the doubt (lost in the post, mislaid, forgot...). If they are in financial difficulties they should approach you as they would their landlord, electric company, etc, and ask for leeway.
I was merely relaying my experience with the situation described in the first post. :: smiles :: Isn't it amazing how writers/authors don't believe they deserve the same consideration as the other bill collectors you mentioned? It wasn't until after I started taking myself seriously that others did the same. Message boards like this one certainly aided in that effort! :D