Yes, Miss Etiquette, I'm using labels.

Dear Miss Etiquette,
I am doing the unthinkable; I'm using envelopes to address my wedding invitations and I don't care what you think. In my defense, I was planning on using a calligraphy type to print onto my envelopes but my dark red envelopes makes it difficult to see the address, so labels it is. You may judge all you want but Martha Stewart doesn't think it's so bad and neither do I.

Sincerely,
Ms. Mek