Is it ethical to refuse a postdoc offer after lab visit

There are no ethical considerations here. They are willing to spend the money to get a look at you as well as let you look at them. Just be honest with them so that you don't seem to be leading them on.

Thinking of it any other way would imply that a small expenditure from them would lock you in. That would be unethical.

Good luck.

There’s nothing unethical about visiting a lab, giving a talk, and expecting to be reimbursed for expenses you incurred for this visit.

The lab will get something out of your visit: the talk and the interaction with other members of the lab. Of course if you promise a talk and don’t deliver that’s another matter.

I agree with the other answers that there's no ethical problem here, and I'd add that, even if these visits had been official job interviews, there still would be no ethical problem. I think it's understood (certainly in my field but I think in other fields too) that people who are finishing a Ph.D. will apply to many places, will go to interviews, might get several offers, and will decline all but one of the offers. No one thinks that, just because they invited a candidate (whether for an "interview" or "just a talk") and reimbursed expenses, the candidate is required to accept whatever offer they might make.