This is only my opinion>> When I sold in a very popular online auction site for 12+ yrs, I tried notes, business cards, and the like for a couple years. It was a waste of my time, money, and computer ink. I learned real quick that sending messages in the message box provided by Paypal and the auction site when the listing ended, was a great place to thank the customer for their patronage and that it would be my pleasure to serve them again anytime.
I had almost 10,000 feedbacks with hundreds and hundreds of repeat customers that never needed anything put in their packages, or the like.
Sending an email note to them thanking them for their business, fast shipping, packing items like a pro, and excellent communication skills is all that is needed. This is only my opinion, and it worked for me like a dream. I was also a Power Seller, so I guess what I did worked pretty good. :=]