Nowadays, every recording/mixing studio offer mastering services. So why should you go through a mastering studio to do it? Simply because the one that will offer in-house mastering services will, most often, process it in the same room (with the same acoustic problems) and would not be able to take a step back on the project (having spend already so many hours on the mixing), not to mention that the workflow is totally different with a different hardware.
With a dedicated studio, you’ll get a new hear on your project as well as a different acoustic, most often able to reveal problem that appeared during the mixing, believing fixing a problem that does not exist, merely the result of an acoustic or a deficient/limited monitoring; or simply by conditioning: working on a project for hours, the problems do not pop the same to your hears anymore.
It is truly an extra service and an extra budget, but it truly transform your project. Give to your project the quality it deserves!