I have been told that the RF Extender sold for the Harmony 890 will also work with the Harmony 880. Does anyone know if that is accurate?

To the best of my knowledge the only Harmony remotes with RF capabilities are the 890 and the 1000. The 880 does not have RF.

You can always use an IR repeater. Not as reliable as RF, but most of the time it's good enough.

Sardo is right, only the 890 and 1000 support the RF extender. You can use IR repeaters but they don't always work 100% ... sometimes they misinterpret the IR sent by the harmony, or it can't translate the IR signals sent by the harmony in sequence fast enough ... but with some fiddling it should usually work.