Thanks, Cadenza.
Sadly it's not a certified copy. Getting the one copy was all we had time for after the wedding, as it usually takes 1 month to get your certificate by mail from New York City. We had already twisted a few arms to get it in person the day after the wedding so we could go to the consulate the day after that. Getting official copies is on my list of things to do next time I am home.
When I say it won't fit, technically it will...it's folded, and in a standard size envelope...I just keep it in a hard plastic cover so that it won't get damaged/ripped, and that won't fit. As the special delivery envelopes are those flexible plastic with no structure, I wanted to avoid having to send it without the cover, but I guess that looks like a risk I will just have to take.
Can't wait until this is all over!!