I think it might be more like a business expense report when you travel. If I'm on a business trip and get a coffee, that's a business expense because I'm traveling for the purposes of work. If I'm working from home and go get a coffee, that's not a business expense. Replace business expense with campaign expense and there are likely rules about what a campaign can pay for and what it can't as well as rules around using personal money for campaign expenses, subverting the necessary disclosure rules.
I read a headline just a bit ago that there were 34 counts in the indictment so I'm guessing what they found spans more than a single transaction.
My understanding is that there is no limit to the amount of a candidate's own money they are permitted to spend on their campaign. So what legitimate state interest could be advanced by laws that nitpick the exact boundary between what is and isn't campaign funding when it comes to the candidate's own money?
Do candidates have to report all their food expenditures as campaign funding-- since they obviously couldn't campaign if they don't eat? :)