The question is should my code be also LGPL, and if not, can I charge people for this?
The LGPL draws a distinction between two kinds of new work that make use of an LGPL-licensed library:
- modifications to the library, where the LGPL-licensed work is modified directly
- a "combined work" (or "work that uses the library" in v2.1), where a separate application uses the library
In the first case, changes to the library must be licensed under the LGPL. In the second case, only the library must be released under the LGPL; the application that uses the LGPL-licensed work does not need to be licensed under the LGPL. (It must follow a few guidelines, though. Notably, it must be easy to for users to make the application use a replacement library if a user wants to substitute the LGPL-licensed library with a different version.)
However, it's possible that you don't even need to follow any of these rules, if your code doesn't combine with the LGPL-licensed work into a single program in either of the above-mentioned ways. If your work is stand-alone program, and only invokes the LGPL-licensed work as a separate program, then your code not bound by any of the LGPL's rules. The distinction between one program with two parts and two separate programs that call one another is a hazy metaphysical and legal distinction; the FSF has some guidelines on how they feel about the subject in their FAQ. If you wish to play it safe, then assume you do form a single program and license your work accordingly.
Finally, you may charge whatever you please for the act of transferring a copy of free software, even if your software is under the LGPL. See the FSF's article "Selling Free Software" for a robust treatment of the subject.