Paper Accessory Query for Vintage Barbie Fans

Calling all and any vintage Barbie fans. I have a tiny Barbie-sized "envelope" that I think is a vintage Mattel Barbie or Francie accessory because I found it in a mod Francie hatbox. It is 3/4" long and 5/8" high. It is one piece, slightly shiny cardboard-like paper. The cursive writing is not legible due to the size - and I do not think that it was intended to be legible. It has two red marks to represent that it was postmarked. Due to its size and my camera skill limitations, it is tricky to show, but here is my best photo of it.

Does anyone recognize it as an accessory to any 1960s Barbie or Francie outfit? If yes, could you please let me know which one or ones. Also, if anyone has an idea of what it would sell for, I would appreciate knowing that. Thanking you in advance.