I've got a customer that wants one of my products that's

I've got a customer that wants one of my products that's international. She says that when she put in her address and setup the shipping option that it gives her an "invalid address" error and won't let her check out.
I've checked and re-checked the settings. I've got the USPS credentials setup, the shipping methods, the whole works, why is it giving her the "invalid address" error?