In the worst case scenario If they dont agree, might take a few people to pull this off. One to have the sign offering free coffee outside, and getting people to install Electroneum on their phones and accept enough ETN to buy a coffee, then someone else inside with a laptop who the guy outside would send people to, who would accept ETN payments and buy the coffee for them. Wouldnt draw too much attention from any manager or staff.
Could ramp it up to three people if we can find volunteers. Sign guy, Laptop guy, then another guy to buy coffees and bring them to the people who ordered them with ETN from the laptop guy.
Would look pretty inoccuous to the Starbucks staff, they might think its just a group meeting at starbucks and buying coffees haha! But in reality, its joe public people comming into starbucks and buying coffee with crypto.
The trick to achieving the impossible, is keeping those who say it cant be done away from the people who are doing it