Thanks. As an update, I wrote Marriott Rewards and they quoted me $40 to park for a week + tax, then $30 for each subsequent week.

I decided to cancel the cert reservation and book a PnF rate since I'd be about $90 more out of pocket, but under the Spring Mega-Bonus I'd get an extra certificate out of it (since I had an odd number of stays already booked). So in effect I could burn a 1-5 cert and $40 or spend $130 keep a certificate and earn another (plus 2K points), I decided to pick option 2.