I think the U++ license states that U++ is the author and that you can use it freely. This means that you can sell TheIDE for 6000€ to anybody. However you should always include the U++ license.
A more complex area is when you modify files. In that case some people does this:
They have three different files:
- 3rd party files: They include the copyright holder license
- Own files: They include their license
- Modified 3rd party files: They reduce the changes to the minimum, include the copyright holder license and a final sentence like: "Modified for XXX (their) project"