1. Try all your points of contact - all team members, AP, etc. If AP is a blanket email like invoices@company.com, look up their AP team online & try to guess their emails

2. Search AP & other points of contact phone #s and call them. People less likely to evade on the phone

3. Try going higher up the ladder than your point of contact. Founder, CEO, dept head, etc.

If none of that works, you can consider naming them publicly, but they won't hire you again. In terms of actually getting paid, it's kind of a hail mary. Maybe not worth it if the outstanding invoices are < $3-5k

I had this happen to me the first time I tried freelancing in 2010. It's why I always request a 50% deposit.
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