I'm trying to book CHS-ORD-DSM-ORD-CHS. Booked over the phone because ORD-CHS was the next morning. Agent said stopovers were only permitted on intl tickets.

here is what ua.com says:

A stopover is permitted on roundtrip award travel only. One stopover is permitted, unless otherwise noted. Additional mileage may be required for Saver Awards within the continental U.S., Alaska and Canada. For travel booked on or after February 1, 2014, a stopover is permitted only on certain roundtrip itineraries. Additional mileage may be required.

is it correct for them to charge additional miles?