That sucks. You need to buy some dress pants. You need at minimum a pair of sand colored chinos, beige dockers, and black slacks. Buy these with your new job discount. Lol
Do you have a link to the shirt you are going to wear? Or something close?
I know some people who do group interviews for entry level part time jobs like this and every one of them says that who gets it an who doesn;t is decided within the first 2 minutes before you even say anything worthwhile.
Dressing properly is VERY important as first impressions are everything.
What shoes are you wearing?