I've been doing the same thing. It really only takes about 10 seconds, and I keep my thank you notes in the same box as my shipping supplies, so I never have to rummage around for them. There's probably a more streamlined way, and when I'm more experienced and more seasoned, I'll probably try to figure it out. But, I'm still new so I'm excited and grateful for each sale, and have trouble not writing a book in each note ;)