Additional Charges at checkout?

My fiancee and I are getting married next month at The Hard Rock Riviera Maya. Everything has been paid for. My future FIL has mentioned a few times that we need to be sure we budget for checkout as he's always had a crazy bill afterwards of 2k+ even at All Inclusives. I thought All Inclusive meant you pay ahead so I'm curious if this is normal. I'm getting worried cuz we're pretty much tapped after paying for a wedding down there. Thanks in advance!