For what it's worth, just as an update to this, we got verbal and written confirmation from Budget that there would be no additional charges whatsoever. We got to the Budget office in Cabo, where they henceforth told us we would indeed have additional charges.. to the tune of $33 (USD) per day for insurance, and then $40 (USD) per day for an under 25 fee, despite using the USAA promo code to waive the fee. Upon discussing it, we essentially felt that we were being intentionally fleeced as young adults. They even almost tried to process the new total - $540, compared to a quoted $45 - without our approval. We had to verbally and repeatedly (not a language barrier issue - I speak Spanish well) say we did not agree to the new total.

We took a cab to our hotel for 500 pesos ($38) and then bussed the rest of the time. Really recommend it.. the bus system could not be easier. Cost us maybe $25 for 8 or so bus rides, including back to the airport.

Basically, don't be like me and think that verbal confirmation from a company will somehow get you past somewhat shady car rental policies.

For what it's worth, I would probably had no problem paying the $13 as quoted (notably, as optional) on the Budget site, but certainly not $33. And taking away the underage promo and then trying to charge $40 a day seems exceptionally shady!