She's not the first - I've had other guinea pigs do this. Granted, I only get a thank you when I'm doling out the really good stuff. Cannoli has yet to express any gratitude.
Right now, Pinni's thank-you food of choice is corn husks. She regularly purrs when I hand it over. I videoed it, but you have to turn the volume way up on order to hear her purr just before the corn husks are whisked from my fingers. The longest purr (and easiest to hear) is first. The other two occurrences are are shorter and quieter. The camera picked up the crunching/munching with no problem!
Do your guinea pigs ever say thanks? Or do they just take the goods and run?