Support Contract?

Film Festival Company A hires Web Company B to put together an on-line film festival website. Web Company B hires me to rip and encode numerous videos from DVD to Flash for the website. I also put together a tutorial document so that Company A can do it in-house in the future. I go to company A's office to go over the tutorial and give them some training in the software used. This was all covered in my agreement with Web Company B.

I also transferred to Company A my license of Sorenson Squeeze (I had an extra) and trained them in how to use it after their initial software didn’t work so well.

Since then Company A had some problems with workflow and certain videos not working, so it would be crunch time and I would encode the videos for them myself and bill per video. That is all fine.

Often times I have helped them over the phone with technical issues related to the hardware and software they've used in ripping and encoding, and a few times going down to their office to help them. This additional support has not been paid and I never had a direct agreement with them about ongoing support. I'm doing it for them and will continue to do so at the current level to keep good relations with the client.

Would it be appropriate to negotiate a "support" contract of some sort with them at this point, per incident? $25 if solved over the phone, $50 if an office visit is needed? In the future I know to have this built into contracts, but since this morphed as me being a sub-contractor to dealing with them directly a formal contract never really came up with Company A.

Hope that makes sense.

Thoughts?