An e-mail:

Thank you for the link.
I’ve been doing that for several years – stopping military personnel in the grocery store, calling out from the car window, etc., and saying, awkwardly(!), “Hey … thank you for your service to our country. We really appreciate it.” If there’s someone who looks like a spouse or family member with them, it’s good to add, “And thank you for all you do, too!”
All it requires of the person expressing such gratitude is a short pause in activity. For some reason, it really doesn’t get less awkward for me, but doing it is important! The recipient always appears pleased, and usually looks surprised.
It doesn’t take much except a very little courage to get past one’s awkwardness, and just think of the kind of REAL COURAGE they need every day!