Don't agree to do the calligraphy for someone's wedding invitations for free, without first confirming how many invitations there actually are.

Because you probably haven't done calligraphy since you were in 8th grade and you don't remember how fucking strenuous it is on your hand, especially when you are doing 160 envelopes and 160 inside envelopes. Yeah, and while your friends are going out, you're sitting at home doing calligraphy.