So what would money order business indicate? There was a money order enclosed in the cover? Why a special postmark for this?
Registered mail was to be marked when received at a post office. The back cancel served that purpose. If you look you can find many types of cancels used for receiving marks, likely just a "oops grabbed the wrong one" or "this is what is in my hand."
Registered mail did go into envelopes (Sealed with OXF at times 1881 was a transitional period) and if the envelope was going only to Shippensburg with the one or more letters from Sexton, no need for a big deal to be done, the manuscript number worked along with the dime stamp.
Saxton to Shippensburg is just under 40 crow file miles and today, by road, 61 miles.
PLEASE NOTE THE CORRECTION TO MY ORIGINAL TYPO: the town is SA
XTON not sExton.