An FLV Editor
 

I have some FLV clips that need to be edited and cut into smaller clips for promo content.

The software needs to add the watermark too :)

Any suggestions where to buy the software for this project?

Have a look at http://www.moyea.com - FLV Editor Pro can do that for you:

"Edit and convert your FLV to video and image sequence. Fresh interface design and powerful edit function like trim, crop, watermark, merge etc."