Meaning of a Japanese coin & thank you note?

What is the meaning of a Japanese coin with a thank you card? Is it cultural? What does it mean? Is it good or bad?
Thank you for your answer.

What kind of coin is it, and where did you get it?

One thing I can think of right off is a 5-yen coin (yellow one with a hole in it)... 5-yen is "go-en" in Japanese, which happens to have the same sound as the word for "acquaintance, getting acquainted, getting connected," and is at times used to express wish to stay well connected, or to thank for the friendship/ having gotten acquainted.

So... if you received it, for example, from someone you met on your travels, that person is thanking you for your friendship :)

...and to wish you good chances (in the future) to get acquainted with wonderful people :)