How can I tactfully decline my supervisor's suggestion and set boundaries? New here. So I just started my postdoc about a month ago (great time to start a postdoc during a pandemic I know). Learning the ropes and trying to kinda set my own path in the lab has been challenging and I find using my voice or voicing concerns to be hard. So my boss mentioned something about me possibly going to use a piece of equipment on another campus about 2.5 hrs away, it would require me to go daily for about two weeks. I don't really need this equipment for anything at the present moment but she has really dug her heels in about staying productive.

My biggest concern is I'm really serious about these stay at home orders and have been kinda putting that to the side to go to work daily. Because I am new I don't want to seem to difficult, but this recent idea just seems like too much in general let alone during these times.

I have a PhD and I thought that would magically make me more assertive alas it has not. Any ideas on how to tactfully set some boundaries especially since it is in regards to my health.
5h commute a day for 2 weeks is not acceptable, pandemic or not. To play it tactfully, I would ask the supervisor to pay for accommodation there for these 2 weeks. This will force your supervisor to:

pay for it, so reconsider whether it is worth it.
if she pays officially for it, she must check whether that trip is authorized by the university considering the pandemic, I know mine has forbidden all travel.
I think it is reasonable to ask for accommodation but would end up being a graceful way of dodging the bullet.