invite question

For girls that have already mailed their invitations - how many extras did you use due to mistakes (wrong addresses, typos, etc.)

I have to send 128 invitations. They come in orders of 25 so I'm ordering 150. I figure I'll have at least 20 extra set. I don't have to order extra envelopes as well do I? I'm planning on having them addressed by a calligrapher and want to make sure I have enough envelopes for any mistakes.

Thanks so much!