Over Limit Package

I sold my copy of the Complete Farside, which clocks in at about 21 lbs.  I rolled into the post office at 5:01 PM and the counter was, of course, closed, so I made my way to the mail chute and dropped the box in.  After closing it I saw this sticker “Packages 13 oz and under only.  Larger packages will be returned to sender. “  I only exceeded this limit by like a factor of 25, I wonder if they’ll catch me.