Welcome to the board!
I don't wear anything under my shorts. I figure that nice big ol piece of chamois is gonna work just as well as undies and I can't even imagine all that elastic over the course of a ride! The shorts are designed to be worn a la naturale!
PS: if you get a chance there's a thread under the open titled "getting to know you"......gives you some laughs while getting to know us, and if you're so inclined tell us more about you - and this trip!!!
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand, strawberries in the other, body thoroughly used up, totally worn out and screaming: "Yeah Baby! What a Ride!"