If there is one thing I’ve learned from these sort of meets is that to never expect anything that isn’t discussed even lightly beforehand. Especially if it isn’t explicitly stated in the ad, and even then I still like some light clarification: “So is the donation only $X? Or are there extras” has been my recent go to.  

Personally, whenever I need to shower or “prepare” I ask ahead of time during setup to make sure it’s ok and have never gotten any pushback. A girl not wanting you to use their shower could be hygiene related and not wanting to share it with someone they don’t know; bad experiences in the past with people leaving a mess; safety issues of having someone they don’t know behind a closed door (drugs, robbery, weapons etc), or anything in between. Whatever her issue was it was clearly important enough to her to miss out on a donation. I don’t think either of you were in the wrong here but I’d recommend to try asking beforehand in the future.

I have found it's rarely an issue if they are traveling and staying in a hotel. With personal homes, it can be hit or miss. Still most are cool with it, and in fact even prefer you shower. But I have come across some that do not want you using their shower for various reasons. It's never bothered me because I'm usually showering for their benefit. But if it matters to you, best bet is to ask in advance.

I've rarely asked to take a shower beforehand and afterwards it's been less than half the time. While it's not a big ask, I think it should be defined prior to the session. I think it's more common with genetic providers, not as much with trans, but only my opinion. The biggest reason for me is if I only have an "hour" commonly at best, I don't want to waste any of it with a shower. I also can prepare best on my own rather than at someone else's place.  

Other girl hiding out in the shower.

I think there have already been some good reasons as to why a girl might not be comfortable with letting you shower in her bathroom, but if she's travelling bear in mind her friend is probably hiding in the shower. It's actually a great tell if you feel like a double. Many times I've asked to use the bathroom and been told no and then parlayed it into a double, sometimes in that very bathroom that I was just denied! Lol.