This is nothing more than a double weight first class letter. The bulky contents clearly create the wrinkles seen at the right end. It might say "due 3" under the stamp if backlighted.
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Most likely sent as a double weight 3rd class cover, so the double deficiency would be three cents
Overthinking and incorrect. Mail paid with at least 1 full rate is charged at only the due amount in this time period. See 1932 Postal Laws and Regulations volume section 763.2
RPO of Wash & Jack ND = Washington, DC and Jacksonville, FL, Northern Division (ND being the Washington, DC to Florence, SC portion of the route)