I've seen those advance booking rates that don't require pre-pay and are cancelable without penalty up to day before res starts. In fact I booked one for Boston later this year.

Interesting thing to note is that generally when I see these and I book them, the rate being advertised for the same hotel later gets updated to say pay in advance, non-refundable. Your original confirmation should have the details even if they change the rate terms later. They won't end up charging your card since you booked under a pay after stay term.