Your hard work deserves the best. If possible always send a wave file. Mp3 files will not give you the sonics that a wave file will.. Try to send your wave file at a higher bit rate and resolution if possible. 24 bit, 48k or higher is fine. Remove any limiters from your master bus. If you are using a mix buss compressor and it gives you “that” sound for your track, leave it on, but make sure your not over compressing. One to two db of reduction will still give you the flavor of your compressor without squashing it. We can not remove over compression. Always allow enough head room. At least 6 db at peaks. don’t worry about not being loud enough we will take care of that in the process