was it an actual PO BOX or was is PMB like at a UPS Store? I have found that people will put PO BOX for UPS Store addresses, and it triggers the inability of the system to recognize it as a valid USPS address... because it's really not but USPS website will tell you it's fine (its not btw...it needs to say PMB Private Mail Box if it is one and not PO Box).

Just a thought of maybe why it's telling you no usps shipping options.