I uploaded only wav file most of the time, and if i had to buy a track, i would always go for wav and convert to the needed mp3 bitrate etc. . Mp3 files are not equal mp3 files, it depends on what software has been used to convert it. I can hear big differences especialy regarding the volume of the converted mp3 file from different converters. Be it iTunes or Logic etc. using the same wav source file.
But you are right, delivering also a mp3 file for the customer to put it quick on a mp3 player is for sure not wrong
SO : I will try to upload both in the future