You don’t need to proactively cancel refundable domestic tickets. The gate agent will cancel your reservation when you don’t check in by the cutoff. The only deadline you need to worry about is that you need to request the refund within 1 year of the ticketing date (not flight date but date the ticket was issued). Once your reservation has been canceled you can either go online and request the refund or call (it’s faster overall to call).

The whole business about canceling before departure only applies to non refundable tickets or international tickets that have specific penalty provisions. I routinely wait until after departure to settle up refundable tickets and have never had an issue.