I usually say thanks, the less syllables the better. But sometimes that’s too casual so I will either stutter through “thank you” or say something like “I’m grateful” with a warm smile. If I’m just out at the store or something like that I usually end up saying “have a good one” to end the interaction if I don’t think thank you is going to work. Always remember that we also communicate with body language and facial expressions. Thank you’s can even be silent. You can make eye contact, put a hand on your chest, and smile. Or just smile.