Well, on the rational side:
A. If they're willing to fly you across the US, there's definitely going to be sales training, they're not going to throw you off the deep end as that's just a waste of investment.
B. Both the training and the job in sales (should?) could increase your social skills and help you face and conquer some of that social anxiety.
C. You need the money. Being in a situation where you
need a job messes up your priorities: You should start looking for a job long before you actually need one, otherwise you run the risk of ending up in one that you hate (again).
D. Two majors, a math and an engineering one, is Effing Awesome, even if it's just college (or are they Masters? Because in that case you're Totally Awesome). You're a prize, a rare valuable specimen, even if you don't like to admit it to yourself. You'll get a lot more offers, so don't worry.
Emotional side:
A. Traveling is scary. But have you ever looked at all the business commuters in airports? Bored out of their skull. Blasé, even. Just pretend to be one, after all, you're just going on a routine business trip. Pretending to be a routine flyer made it even fun, for me.
B. You're young, malleable, cheap, well educated, and intelligent. You're a good investment. Keep that in mind. Also remember that you can quit at any time, and just start a new job elsewhere. This doesn't have to be a Major Life Decision.
C. Selling stuff... I'm not good at it either. Perhaps pretend that you're the potential customer. What does someone need to say for you to say yes? Well, say that