I think that's probably useful to have in your default contract, but IMHO the people who are adamantly refusing to pay you $5,000 are very unlikely to have a change of heart when you inform them it's $5,100. Consider that debt collectors typically offer to settle for potentially far less than the original amount owed.

One of the most surprising things I found when I started working in B2B is the percentage of clients who simply don't pay their bills.

Like tptacek said, I think that's just the way it is and, at least at a certain scale, the only reasonable approach to hire people to do collections.