Cannot mix cash and points on a single reservation. Cannot BidUp to roomettes on points reservations. Cannot modify a points reservation with cash to upgrade, or vice versa.

There are two workarounds. If your itinerary includes multiple segments, split the segments into two reservations, one paid for points, the other cash. Note this is not good to do on same day connections from long distance trains, since it endangers the connection guarantee. The other workaround is to buy points for cash if you are close on points.