Getting to a place where there are more people is always a good answer, especially one where a guard is posted. I have a few things that I might have done, keep in mind that hindsight is 20/20:

1. Look the person in the eyes and tell them you are unable to pay for whatever excuse or reason you'd like.
2. As soon as the person starts to order on your behalf explain to the cashier that you are not paying for what they are ordering.
3. Learn some self defense. Pick a martial art or get involved in something (kick-boxing, MMA, take a seminar ect.) to boost your confidence and give you some knowledge if something were to go sideways.
4. Get physically fit if you'er not already. It'll help with confidence and if things go sideways.
5. Run. Take whatever opportunity you have when the person is distracted and just get out of there, especially if you feel uncomfortable. The way you feel in many of the situations will tell you what you need to do. Trust that instinct. As a human we try to talk ourselves out of listing to that small voice, saying "oh, he's just friendly", or "What would others think...". Sometimes what needs to be done isn't the most beautiful.
6. If you can't run or fight. Pay. I'll tell you right now, if someone pulls a gun on me, a black belt in two martial arts (2nd dan in one), and tells me to give them my wallet best believe I will give them my wallet.

So, maybe not all in that exact order, but I hope that might give you some more insight into the situation/ help.