If you need a LAS-MIA one-way ticket in F/J on AA I can make a suggestion. Historically AA has offered relatively inexpensive F/J fares between LAX/LAS/SFO and CUN. You can fly via MIA, where you would get off, and set the date for the MIA-CUN segment to one day after your arrival in MIA, so you wouldn't have a problem with picking up your luggage at MIA. Currrently you can get a one-way LAS-MIA-CUN for about $550, albeit on the dates I checked this fare doesn't seem to be available on the LAS-MIA non-stops, so a stopover in DFW may be required. On the bright side as a BA silver you'd get to use the AA lounges at DFW, including AA's fab Flagship lounge.