Why is it not "pismo"?
"Pismo" may indeed be some formal, official letter, but it seems safer not to add it, as it's a false friend with Russian - where it just means any letter. And there's nothing wrong with calling an official letter "list".
Isn't piszesz also correct as that also means "you write"
Yes, it's the singular 'you' write. "piszecie" is plural.