Try to keep the master bus free of any processing in your mix, especially dynamic processors (compressors / limiters), unless there's a specific sound of a master bus processor that you really need.
Make sure the song never exceeds -3db peak on the master bus.
The file you are sending us must be an uncompressed audio format, such as .WAV
Avoid using conversions, so keep the file at the same resolution used for mixing. For example, if the song was mixed at 24-bit 48 kHz, we expect to get a 24-bit 48 kHz audio file.
Feel free to add any notes or reference tracks and don't forget to specify your distribution environment, such as streaming, cd, or vinyl, when creating the project.