I think all modest people have trouble asking for compensation, especially if compensation is not their only motivation for the work. Many of us do work we love, and the compensation, though necessary, is not the prime motivator. Stating the compensation makes it seem as if we are totally pecuniary.
But, somehow we have learned to overcome that, to avoid mistakes, misunderstandings and resentments from our customers.
It took me a long time to learn this, after many misunderstandings, that the best way is to come to an agreement up front, then go do the job.