And definitely a fan favorite:

"Hi [X], I'm a customer of [PRODUCT]"

Also ... develop a summary pitch. Sure it would be great if they could check out your aff pitch page and get the FULL story, but they likely won't make the time. (I rarely do). Summarize it.

And talk of win-win-win. Namely what do their subscribers get and what do THEY get and what would they need to do in trade...

Also, a solo mailing may not be an optino, but a banner rotation in their membership, a banner on a thank you page, several facebook status mentions, a few tweets, a blog post, etc... any of that should be welcomed exposure (especially if you then leverage that to land other JVs.