Hello Sylvain,
Burgsteinfurt - Münster was send as a "Portopflichtige Dienstsache", what meant official letter to be charged. Usually simple letters were paid from the sender with 1 Silbergroschen, or had to be paid by the addresse with 2 Silbergroschen. When the sender was a public authority, there was only the rate of franked letters to be paid by the addressee. The big blue "I" was for one Silbergroschen (= 2 1/2 US Cents to give you an impression of the value).
When it was sent back, it was franked as a simple private letter.
Those letters are/were no rarities, but they are nice to show and when you look for them, it will take some time, to find one. This one is pretty and with its little postal story inside certainly a good buy. 
