Did some part 91 on the side in a Citation several years back. I was told by my ACP to put in the letter that there would be no conflicts with my legality for Fedex flying and that I knew that my primary focus would remain on my job at Fedex. As long as I put the company first, he said there would not be any problem. Had the letter in hand within a week. I did have to write and receive a letter each year.