You can use Viur for free by complying with the Lesser General Public License
» The GNU Lesser General Public License (formerly the GNU Library General Public License) or LGPL is a free software license published by the Free Software Foundation (FSF). It was designed as a compromise between the strong-copyleft GNU General Public License or GPL and permissive licenses such as the BSD licenses and the MIT License.
The main difference between the GPL and the LGPL is that the latter allows the work to be linked with (in the case of a library, 'used by') a non-(L)GPLed program, regardless of whether it is free software or proprietary software. The non-(L)GPLed program can then be distributed under any terms if it is not a derivative work. If it is a derivative work, then the program's terms must allow for "modification for the customer's own use and reverse engineering for debugging such modifications." Whether a work that uses an LGPL program is a derivative work or not is a legal issue. A standalone executable that dynamically links to a library, through a .so, .dll, or similar medium, is generally accepted as not being a derivative work (as defined by the LGPL). It would fall under the definition of a "work that uses the Library".
The following is an excerpt of paragraph 5 of the LGPL version 2.1:
A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a "work that uses the Library". Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License. «
Essentially, if it is a "work that uses the library", then it must be possible for the software to be linked with a newer version of the LGPL-covered program. The most commonly used method for doing so is to use "a suitable shared library mechanism for linking". Alternatively, a statically linked library is allowed if either source code or linkable object files are provided. One feature of the LGPL is that one can convert any LGPLed piece of software into a GPLed piece of software (section 3 of the license). This feature is useful for direct reuse of LGPLed code in GPLed libraries and applications, or if one wants to create a version of the code that cannot be used in proprietary software products.(Extract from Wikipedia)