Below is a scan of a DLO ambulance cover dated 22nd June 1955. There are two 10 cents postage due stamps tied by a faint pencil scribble.

5 cents was the cost of the DLO service. If the original cover had been posted without a stamp then there would have been a 10 cent postage due charge making a total of 15 cents
Can anyone suggest why the 20 cent charge.
AQ