I could send you a short AviSynth script to do this if you send me a 4 second clip from the video. Unfortunately you would need to install AviSynth and VirtualDub and probably FFMPG on a windows based machine to use it. Hopefully some one will have a solution for Mac. I don't see the quality going down by doing this. All the frames are still there you are just changing the FPS. Always try to test videos with altered FPS in a few programs. I once made one that played fine on my computer but my client had the latest and greatest version of Windows with their newest media player version and it was "improved" to not play the odd frame rate.

I think this could help you video - freeware tool to change playback speed of avi file - Super User

If you are going to be making more movies, I would opt to get Final Cut Pro.

Final Cut Pro actually has two options for changing the speed of your video - constant, which slows down or speeds up the entire clip, and variable, which allows you to change the speed of the clip within the clip itself.

FCP is a Awesome program, I still have hope that Apple will do the right thing and update it, but you never know for sure, ) what are you using to distribute your video? just curious as platforms can effect the way a video is received on the platform

Im had same problem and I solved it thanks to ionutcib - thank you.

I know thats not your problem, but 20 minutes sales video is very long. You have to impress viewer in such a shorter time.