USPS Will Not Accept The PO Box for Business Unless There Is Additional Evidence
When a USPS location does not allow a business to be selected for the trust [see sample application form below], one can show the trust indenture to prove it is a legitimate trust/business organization. According to their PS1093C application, USPS doesn't have any requirement for a business license to prove something is a business. All that is required, per their instructions on the form, is the 2 forms of identification.
NOTE: Should all else fail, simply select PERSONAL for the use and list the trust name under BUSINESS NAME(S) to identify that a trust or entity separate from you will be accepting mail at this location.