I'm not sure there's anything I really could have done. I was working on 30 day terms (quite common), and I'd worked for them with quite prompt payment for over a year. I probably had ~12-15 paid invoices from them for similar amounts that were paid within 7-14 days of issue.
I stopped working for them upon the issuance of the second invoice in question - so I had two invoices outstanding at that point, 1 that was 15 days past issue (but not contractually due) and 1 just issued.
Do people in multi-invoice contracts really stop work until they have proof of remittance? I've never heard of such a thing - that sounds crazy to me.