Hi Gizake,
All royalty-free websites have different T&Cs, but what you are suggesting probably breaks those rules.
While it's unlikely, if the website finds that you are illegally using their music, you could be liable to prosecution but most likely your video will be removed on copyright grounds. $20 is not much for a peice of music that you can use again and again, and if you just download it without paying, the composer doesn't get the money, which is unfair!
If you send me a URL, I'll look into it a little further.
Hope this helps,
Ben