The dimensional-weight calculation guide
uses packed outer dimensions and distinguishes carrier, service and account divisors before comparing the result with actual weight.
Before changing alignment settings, the ZPL-versus-PDF reference
checks whether the application and printer support the supplied file. PDF and ZPL are different printing paths, not label sizes.
For a qualifying rectangular USPS DIM calculation, round each outside dimension up; above 1,728 cubic inches divide by 139 and round the weight up, subject to the 70-pound DIM price cap.
Compare Small, either Medium, and Large Flat Rate boxes by usable inside dimensions, then price the actual shipment against regular or eligible commercial cubic service.