What you're doing has to align with their mission and goals.

You have to talk in their language, not yours. If they're about feeding the homeless, then that's how you should phrase how your service helps. If they're about spreading awareness of an issue, then that's what you talk about and frame your service to support.

An NPO is going to be concerned about you trying to borrow their credibility. They don't know you. They don't want to be associated with someone who could turn out to be bad. This is the trust hurdle you have to overcome.

So your service has to be marketed as useful specifically to them, and you have to be seen as not trying to leverage their credibility right off the bat.

I would look to develop more reasons why they should be excited to partner with you. One probably isn't enough.

This post about my own NPO experience may be helpful to you: http://www.salestactics.org/sales-for-npos/