If you are homeless there are ways to get postal service from your local post office. Postmasters can grant free PO boxes at their discretion (1). You are also able to take advantage of general delivery (2). General delivery is available even for those who are not homeless but merely without a permanent address.

1. https://faq.usps.com/s/article/Is-there-mail-service-for-the...

2. https://faq.usps.com/s/article/What-is-General-Delivery