The OP does not mention where his flight to CUN originates but, if it is MIA-CUN, let me just say that IMHO, that segment is not worth an upgrade (not with a sticker, and under no circumstances with a SWU). It's a short flight (1h30m) with relatively limited service in F. I upgraded myself and a workmate once at check-in in MIA, and only remember the occasion as a waste of time, effort, and stickers. My colleague was equally disappointed. Perhaps the OP is flying out of a different airport (i.e. not MIA), honeymooning, etc. and wants to reward himself and his travel companion with F seats, in which case upgrades become a completely different factor to consider.