Chasing down late payments from apartment complexes

I've been running a small plumbing crew for about 8 years now, and dealing with collections on bigger jobs is always a pain. Last month we finished a full repipe on a 20-unit apartment complex using PEX-B throughout, but the property manager is dragging their feet on the final $4,200 invoice. We're in Texas so these commercial gigs can be lucrative but the payment terms stretch out to 60 days sometimes. I started sending weekly reminders via email and even called their accounts payable twice, but no dice yet. What I do is keep detailed records of all the work, like photos of the manifold installs and test results, to back up any potential lien filing. If it gets to small claims, I've had luck there before without a lawyer. Anyone else got tips for speeding this up without burning bridges?

File the lien notice right away if it's past 30 days; in my experience that lights a fire under them without much hassle.