Round trip deviation ticket--FDX

First off this is a FDX question. Sorry if it has been covered.

I know a deviation ticket has to be within three days of an activity. What happens if I buy a roundtrip ticket online (not through Corporate Travel)? Is that okay as long as the first flight is within three day? What about the backend of the roundtrip ticket?

Looking for feedback from those with experience buying their own tickets. Thanks in advance.