Personally, I'd say you should just be completely honest. I would always overthink everything when it came to girls (still do), and I'd be so afraid of getting rejected, that I would not make any move at all. Say something like "I really suck at this... I like you, and would like to get to know you better. Would you like to go out on Friday?" Something to that effect. I have not heard her tone, nor exactly what she said, so I can't give anything else other than that.

No matter what she says at that point, it will be in your best interest. I can't speak for all girls, but for the ones I have had a crush on at one point or another, they have been decent human beings. If she is just pulling your leg, then she isn't the kind of person you would be happy to spend time with, anyways. It could also be possible that the last letter indicates she is embarrassed, and is afraid to say anything.

Bottom line: tell her honestly that you are bad at this stuff, but you would like to spend more time with her.
Adjust where deemed fit.