Most Mastering facilities offers FTP (file transfer protocol) services to their clients so they (clients) can upload their songs. The clients have their own accounts and personal space to their files
You would have to upload your stereo mix files to it (wav, aiff etc.)
You can get your Mastered files back by FTP again (download) or get the Master CD by mail.
Cost: depends on the studio/engineer. By visiting their website or calling them, you will get the answer to this question.