I just have a company in Poland, sign a contract with US company and send them invoices every month (which, to my knowledge, they don't really always even need?

I just have a company in Poland, sign a contract with US company and send them invoices every month (which, to my knowledge, they don't really always even need?). I don't know how it's set up on their end. No one yet has asked for a US-based entity. As to finding them, I've found some here on HN in Who's Hiring thread. One of them (I'm a notorious job hopper, so there were many) found me on LinkedIn.