[GhostVII]

I think a better strategy would be to send an email as soon as the payment was late, saying a late fee would be assessed in x days if payment is not received (or just include the late fee in the contract, I suppose). Attaching an arbitrary late fee without any notice seems strange to me, but I don't do freelancing so I guess I don't really know. It just seems like any fees should be documented somewhere, and people should be allowed to accept/reject them before entering into a contract.

[phonon]

In NYC, under the "Freelance Isn't Free Act" (which she mentioned in her emails) paying later than 30 days gives an award of double damages (twice the amount owed).

https://www1.nyc.gov/assets/dca/downloads/pdf/about/Freelanc...

https://www1.nyc.gov/assets/dca/downloads/pdf/workers/FAQs-F...

https://www1.nyc.gov/assets/dca/downloads/pdf/workers/Court-...

[Traster]

I don't work in the area, so I know very little about it and this may seem naive - but I constantly hear adverts for things like Quickbooks - where they'll automatically send invoices and chase up. As I say, I know nothing about the area, but those products are around $20 a month which seems like pretty good value and would leave your clients under no illusion about what they to pay when and with what penalties.