It's not unusual for some authors to ask you to post their copyright in return for a free mod. If you paid for the mod, you should go to their support forum and search around to see if they allow you to remove it.
Also be sure to search the mod's thread here to see if someone hasn't already asked the same question.
To be honest with you, this kind of sounds like you're trying to decide what you can get away with, and that doesn't seem like an appropriate discussion for this forum.