is a type of paper with a very smooth surface. During the manufacturing process, heat and pressure are used to produce a smooth finish.

It was ( and is) expensive paper to use,even in JA's time when msot paper was a luxury. Of course,bearing in mind who wrote the letter..... ;-)

Do you notice,BTW,that when Hill hands over the letter ,Jane opens a sealed envelope.Then, as she is reading the letter there appears to be an address written on the rear of the letter to suggest it was one which was folded and then sealed ,as was usual practice in this era.Even though it had just been removed from an envelope.

The envelope, though known in the early 18th cnetury( Jonathon Swift mades one of the first mentions of it, I believe) did not come into genreal use until the penny post was introduced in the1840's.

Seems as if someone was unsure of the practice adopted at the time and decided to opt for both.