Long-time Us Client Suddenly Refuses To Pay ~$28k, Calls Years Of Work "unauthorized." I'm Overseas. Worth Pursuing?

Throwaway for obvious reasons. I'm a solo software developer based outside the US, and I'd like a sanity check from people who've dealt with non-paying clients.

Location: Arizona, United States

The setup:

• I worked with one US client for about two and a half years as an independent contractor, billing hourly in my own name.
• Over that time I issued 80+ invoices and every single one was paid without any dispute. Totally normal working relationship — regular meetings, they assigned the work, I delivered it, they paid.
• There was no signed master contract. It was a long, consistent course of dealing: invoices, meetings, deliverables, payments, repeat.

What happened:

• I did a large block of work over several months. When I invoiced it (~$28k total), the client stopped paying.
• They've now terminated the engagement and rejected the outstanding invoices in writing, saying the work was "unauthorized" and that "nothing is owed."
• This is after 2.5 years of them assigning me work the exact same way and paying 80+ invoices for it. Nothing about how I worked changed - only that they decided to stop paying.
• I have full documentation: all invoices, records showing the work was requested and delivered, and a complete reconciliation.

My constraints:

• I'm overseas. I can't afford cross-border litigation or US lawyers.
• There's no signed contract, though I understand a consistent 2.5-year course of dealing may count for something.

Questions:

1. Is this realistically worth pursuing, or should I write it off?
2. Does "the work was unauthorized" hold any water when they assigned it (on record) and paid for the same thing 80+ times?
3. Anything I'm not thinking of?

Appreciate any honest takes - including "move on," if that's genuinely the smart call.