Finnish greeting in writing letter for a friend

[Venpa]

Can someone suggest me Finnish greeting while writing a letter for a friend? And how to end the letter with a Finnish greeting? Not too formal but a friendly greeting.

[suruvaippa]

(disclaimer: not a native speaker, so anyone is free to correct me on any of this)

For the opening greeting, the most typical would probably be as follows (I'll name the recipient Aino as an example):

Hei Aino!
Hei, Aino
Hyvä Aino,

The first two are, as far as I understand, informal, while the third can be used in either formal(ish) or informal contexts.
If you're writing to a particularly close friend, you can also use "Rakas Aino," which would be the direct translation of "Dear Aino," but I'm fairly sure that Finns reserve this greeting for people they actually consider dear, instead of defaulting to it like we Americans do.

The standard greeting at the end of a letter is:

Terveisin
Minä

Note that unlike in English, there is no comma used after the greeting.
Again, if you're writing to a close friend, you can also use "Lämpimästi" ("warmly") or to even closer friends "Ystävyydellä" (not sure if there's an equivalent in English. Literal translation is "with friendship")

[Woods]

https://forum.unilang.org/viewtopic.php?t=24032

[Linguaphile]

For closing a letter I've also used "Parhain terveisin" (best regards/wishes/greetings) and "Kaikkea hyvää" (all the best).

These sites have lots of good phrases for letter-writing:
Finnish Phrases: Personal - letter
Writing Letters and Emails = Kirjeiden ja sähköpostien kirjoittaminen

[Naava]

For closing a letter I've also used "Parhain terveisin" (best regards/wishes/greetings) and "Kaikkea hyvää" (all the best).

Sure, but those can sound a bit formal to a friend.