I don't carry a spare. Despite trying, I've never managed to damage a shifter. And I use the supposedly delicate/finicky 10-speed STI and DoubleTap shifters. I don't make the clamps super-tight so the levers tend to rotate, rather than break, in a drop/crash. If I were worried about the mechanicals, I'd replace the shifters before leaving on a trip.

Worst case, the bike becomes a single-speed until you reach the next town and can get repairs/replacements.