One link. They open it on their phone. You see Viewed, then Paid.
Nordwind Contracting
INV-1042 · Due Sep 10
Send a link, not a screenshot.
Client opens it on their phone. You see Viewed. No account for them.
They confirm. They pay.
You get a timestamp, not a WhatsApp «ok».
Due date does the chasing.
Unpaid past due becomes Overdue on its own.