Via Delta.com, purchased vacation to Cancun on Delta Vacations. Prior to purchase, I put trip on "24 hour hold" and called Delta Vacations to confirm that I'd be able to use Delta Regional Upgrade certificate on the return flights that were booked in an upgrade eligible fare class if upgrade space was available.
I may be wrong here, but should you have not applied a GUC for this flight instead of an RUC since Mexico is an International destination?