I have been thinking about this, and I wouldn't know how to fit a bra for a transwomen or man with gynecomastia.
Some of which I won't go in to bc lurkers but i.e. the bone structure of the rib cage is different between men and women, as are proportions. With developing tissue you need to be careful to avoid damage.
Its not discrimination, on sex or gender reassignment. Anyone can buy a bra. Even if it was for their own sexual gratification, fine as long as they don't involve me without my consent.
TRAs see us as so hateful. I would just want a quick word with the staff member, and explain why many women / girls would not want to be approached.