Do you have any limiting or clipping on your mix? If so, is it an integral part of the sound or included for client reference? If the latter, please consider removing any limiting or clipping prior to submission, or providing both versions.
Sibilance is a common issue in mastering that is best dealt with in the mixing process. Often times attempts to reduce vocal sibilance are done so at the compromise of something else in the mic (cymbal sheen, the high-end snap of a snare, the air, and sheen on an acoustic guitar, etc).
Listen through your track and observe if the “s” on your vocals are offensive or unnaturally loud. If so, consider using a de-esser on the track(s) to tame sibilance. Be careful not to remove too much as this can be equally detrimental. You’ll know if you’ve gone too far because often it will be perceived as having given the singer a lisp in their performance.
If you have any other processing on your master bus and are unsure about whether you should leave it activated on the audio file delivered for mastering, consider sending two versions of the song: one with any additional compression/EQ processing that the client approved on their mixes, and one with no processing on the master bus
Listen very thoroughly to all tracks to ensure there are no clicks, pops, or other unwanted artifacts.
Leave some headroom! Your mix should be well clear of clipping over 0dBFS.
Export the song(s) as WAV or AIFF file(s) in the bit depth/sample rate you recorded in.
For example, if you recorded in 24-bit/44.1kHz, export the file at the same resolution.
Please name the song files exactly as you’d like them to appear (spelling, capitalization, punctuation, etc.) along with the album sequencing. For example, your file name should look something like this example of a three-song EP submission:
Artist Name - Song 1
Artist Name - Song 2
Artist Name - Song 3