Dunster in the Taunton Fifth Clause Post

The entire below was sent from Dunster to Westminster in December 1815, at which time Dunster was in the Taunton Fifth Clause Post.  The entire weighed over an ounce so was charged at five times the single letter rate.  When I first wrote it up I could not quite understand why it was charged five times the General Post rate from Taunton to London (5 x 10d = 4/2d) rather than five times the rate from Dunster to London (5 x 11d = 4/7d), particularly as the DUNSTER/183 mileage handstamp implied that it was over 180 miles from London.

Then I realised that the sender of the entire should have pre-paid the Fifth Clause Post charge from Dunster to Taunton, leaving just the General Post charge from Taunton to Westminster - which is what we see.

From other Taunton Fifth Clause entires, the rate for the Taunton Fifth Clause Post seems to be the same as for the General Post - however that may not be correct for multiple sheet or weight items.

Another puzzle is why the "DUNSTER / 183" handstamp (in use 1810-1816 according to the BCC) was issued when Dunster was under the Taunton Fifth Clause Post rather than being in the General Post.  My hypothesis is that, at least for single sheet letters, the rate that had been agreed for the Taunton Fifth Clause Post was to match the General Post rate.  This would mean that the mileage, as per the Dunster handstamp, could be used to calculate the total postage charge, and this could be marked up and charged to the recipient (as it would match the General Post rate based on distance that had been approved by Parliament)

However this does not work for multiple sheet or heavy letters.  The Fifth Clause Post charges are said to not increase with the number of sheets, or with weight unless the weight was greater than four ounces.  In this case that might mean that the extra Fifth Clause Post charge from Dunster to Taunton was just 1d.  As the total (Fifth Clause plus General Post) charge wouldn't match the legally-approved General Post rate it could not be marked up and charged to the recipient (the Post Office would not be able to justify it if the recipient queried it as the Fifth Clause rate had just been agreed with the local inhabitants rather than being passed by Parliament).