Do not call. First, it gives them a good opportunity to cancel since you aren't literally standing at the front desk where you could potentially cause a scene in front of other guests. Second, even if they do "confirm" it, you aren't actually guaranteed anything -- some underpaid front desk employee saying "si, you have a reservation" over the phone doesn't mean a manager won't catch it later, say the front desk employee was wrong, and try and cancel the booking anyway.

I agree with the other commenters it's a toss-up ... I've had these types of bookings both honored and denied in equal measure. If you wanted to go to Tulum anyway, book a (semi) refundable backup option. If you are only going because of the deal...get ready to gamble.

EDIT: By the way, I wouldn't beat yourself too much over not choosing the prepaid option. While it would've been the better choice, it still wouldn't have eliminated the possibility of the hotel trying to charge you more on arrival, or claiming Expedia was wrong somehow.