Is ending with "greetings from..." a good informal way to end a letter ? And is "Best regards" more formal, although not as formal as "Yours sincerely" ?
I wouldn't use Greeting from- to me, it sounds like something you could write in a postcard with the place you're staying. I would use Yours faithfully/sincerely formally, Regards for less formal letters and emails, and Best wishes or Love informally. Others may sign off differently.