I will chime in that I've tried Guayusa (supplied by Runa) and I was quite impressed. It's somewhat similar to Yerba Mate but different enough that I think it wouldn't be overkill to carry both.
My big question, regarding caffeinated hollies...who is going to be the first company to sell Yaupon? The shrubs are available but I've never seen it for sale. Perhaps they're put off by the scientific name, Ilex vomitoria.
Here's a suggestion: get the tea and herb industry together to petition for a scientific name change (the name is not accurate, as it does not induce vomiting; it was simply historically consumed in certain cultures as part of a drink containing other ingredients that did).