Net-60 with a new client is a rough lesson, but I’d keep the next step very specific rather than making it emotional.
I’d send something like:
“Hi [Name], following up on the outstanding invoices for [project/work], totaling $4,200. These were issued under Net-60 terms and are now past due. Could you confirm the payment date today? If there’s an issue with the invoices or deliverables, please let me know so we can resolve it.”
The main thing is to stop leaving the next step open-ended. Give them one clear action: confirm the payment date or identify the issue blocking payment.