Replacement shop referrals

[NuVueGlass]

Here's a question for those of you that have a referral program setup with any replacement shops. Do you have an agreement form to guarantee that you receive your referral fee. If so, could you pass along any ideas on wording. I have affiliations with a few shops on a handshake agreement but I recently got burned by one. Of course that ended our relationship, and I want to make sure that this doesn't happen again. There plenty more shops to work with, and I like the informal, friendly, non binding, handshake agreement but I also want to make sure that they understand that I mean business.

Thanks for your help.
Steve T.