Flap steak? Not familiar with that cut.
I usually use skirt for fajitas.
It's a tremendous cut of beef. Similar to a flatiron and very tender.
https://www.google.com/amp/www.sfga...best-kept-secret-Seldom-seen-flap-3255048.php
Funny: I always read how flat iron has become expensive and hard to find. That isn't the case at all around here. They always have it at Ingles and Kroger in shrink-packs for around $8/pound. Simple marinade and seared on a hot grill for 4-5 min/side then rested and sliced thin, I swear it's the best bang for your buck in the beef world. Much more tender than flank or skirt.
Take'm out Jeff