You might not ever need to use a personal pronoun if you’re only ever going to have fleeting interactions with service personnel.
Right, because the topic marker (in my case: ordering food, buying tickets) will be obvious?
I think so, yes. From what I have heard in class, though, わたし might be the best fit. If you really need to use it. My 45yo teacher generally uses 俺, but switches to わたし in more delicate situations (like a native speaker visiting a class).
I guess my question’s broader than that, then … because no matter what I say, I’m going to have to use a verb, and I’ll still have to choose between casual and polite. Is it weird to be polite to service personnel? I’m Canadian, we’re famous for it. Or is that kind of politeness reserved for Japanese bosses or meeting the Queen, and would seem strange with my ordinary interactions? (At home, at Starbucks, I say “Hi! I’d like a tall coffee please, with room for cream” … I don’t say “tall coffee” and I also don’t say “if you could do me the very great goodness of preparing a 12 ounce coffee, I’d be ever so grateful”) …
My understanding is that it’s normal to use keigo with acquaintances, service personnel, and other non-familiar people at that level. It’s not super formal/honorific, but still conveys politeness.
Thanks so much! I will prioritize memorizing my polite forms then, and save the casual for when I unaccountably suddenly have Japanese-speaking friends or family 
Definitely -masu form is the way to go! Some degree of sonkeigo or kenjougo might be expected in certain situations, but as you are a learner I wouldn’t concern myself with that too much yet.
There is a difference between formality and honorific language, btw. Not sure how to explain it, though, especially in English.