For people who aren't aware of actual divisions and movements within feminism (tip; has very little to do with tumblr, which has every group you can imagine along with a fair few you probably couldn't dream up after a hit of LSD or five), there are a substantial subset of modern radical feminists who do not accept transwomen as women. These are sometimes referred to as Trans-Exclusionary Radical Feminists (TERFs). You generally see the most vitriolic transphobia coming from this group and I'd fully expect them to jump all over this as a way to attack all transwomen.
Fakedit: Yeah, there we go.
I say subset of modern radical feminists because there are some radical feminists who accept transwomen and speak out against such groups, while the writings of at least some early radicals (seen as founders of the second wave) were at least less strident on that point (take
Dworkin, although her views on gender were radical in the most literal sense anyway).
Last things first: Your example is a very good one. Women would (probably) face no more discrimination (in the form of drawing flak from their surroundings) for taking the Parcelforce job than for taking the Amazon one. If a woman - or women in general - choose not to pick the Parcelforce job, it's their loss.
But the job being effectively gendered by the management and staff means that women are unlikely to take it, even if they could do it. It's that gendering of the job, by the staff and society at large, that keeps them out. Personal preference plays into that, but such preferences are more shaped by society than you seem to be accounting for.
The cultural stereotypes play a substantial role in keeping the women out of those jobs. The Amazon counterexample was meant to demonstrate this. It lacked the baggage of being an obviously 'physical' job and wasn't in an established male dominated field, so, despite being broadly similar, recruited far more women.
The way I see it, a woman going for the Parcelforce job or any other traditionally male role would have to get past three obvious barrier;
1) Potential discrimination by management/recruiters who don't believe a woman would be suited to the job.
2) Similar discrimination by the workforce, making them unwelcome or uncomfortable.
3) Personal expectations of either of the above that put them off applying.
3 is significant here. If a field has a reputation for being male then women are unlikely to apply simply because exposing yourself to potential discrimination is not attractive. Saying women choose not to apply for or enter a traditionally male field may have more to do with that field's reputation and expected treatment of women than the women's actual preferences or desires as far as work goes.
Which is to say that both 1 and 2 are explicitly harmful, but even the cultural perception that they should be expected for entering a field can have a harmful effect on women by pushing them out of a field even before they apply. Working to actively and visibly reduce and oppose explicit sexism while also actively courting and encouraging women in those roles is the only real way to address such issues.
Which is all a longwinded way of saying that pretending you can create hard and fast rules for treating someone based on gender is in itself a harmful idea, even if it doesn't seem so on the surface.
And the whole holding open a door thing confuses me. Do you not hold them open for men? Where is that acceptable? Someone letting a door close in your face has always been seen as fucking rude as far I was aware. And what exactly is it about a woman that makes holding a door for them more required that holding a door for a man?
And that's before going into the fact that making such calls based on genders means you have to assign a gender to everyone to know how you are supposed to treat them. That is problematic in itself.