It started to rain while I was leaving the grocery store and I intentionally left my daughter in the car to wheel the buggy to the front, while thinking about this thread.
I don't know how many times a person has been struck with lighting while pushing a shopping cart, but I'm thinking it's close to non-existent. If you have that much fear you should wait in the store for the storm to pass. Typically lighting strikes the highest point in the area, so a lighting bolt would be aiming for you more than it would the shopping cart. Not to mention it would be more likely to hit a light pole in the lot, than a person pushing a cart. But anything is possible, I know of a runner who was jogging on a track during a small storm and he was hit by lighting, it blew one of his sneakers right off his foot when it went through him.