It is a reply as you said the letter is on the inside and I could read it as the present is wrapped!
As it is Christmas I opened your present!
It was lovely, thank you! Hehe, it was very nice!
And Jenny's was too, it's really interesting, I've been reading it, it's very good!
Funny, as in the reply to Jenny's letter I mentioned something along the same lines!
How was your Christmas?