Is there an etiquette followed for asking about a possible (even if small) honorarium for invited talks? These are not talks at conferences but a stand-alone event on campus, open to the public, and rather stressful. As an early career academic I know this is quite important to have on a CV, and would accept the talk regardless, but would like to ask whether they would be able to pay me for this labor. Is there a way to do this kindly and courteously?
[All of the following written from the perspective of mathematics, sciences, or geosciences where departments generally are not sitting on piles of corporate donations and industry funds.]
If you are talking about seminar talks at other universities, then the answer is that "no, there is no procedure for asking". As a general rule, giving seminar or colloquium talks at other places is part of your job duties and not a side business. It is generally expected by your department that you do it. If you do it only because someone pays you money for it, then it becomes a side business. If you consistently think that giving seminar talks is high-stress, then you need to either reconsider whether you're in the right line of business, or just not accept any invitations.
Now, some universities or departments will voluntarily give you a honorarium. If they offer that, you can accept it with gratitude. But most departments will not offer one, and asking for one is sure to make for a very awkward situation; it is also sure to make you look bad in the community.
All of this is of course separate from reimbursement of actual travel costs. You can expect that these are paid for, i.e., that you do not make a loss by giving a talk.