What should I wear to an interview when I was told "Wear whatever I want?"

I have a job interview coming up at Walmart eCommerce and part of the information I was supplied with told me to "wear whatever I want." Now, I take this to mean the suit I was going to wear would be too formal for a pseudo-startup culture, but I'm going to go out on a limb and guess jeans and a sweatshirt would not help my case. I was thinking dress pants and a rugby shirt (the best I can do here at college), but has anyone had experience interviewing for casual environments? Can you give me some advice?

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