All-black sneakers usually never fail at work, and they don't draw attention. No visible logo or stripes means nobody looks at your feet twice and asks why you're not wearing dress shoes at a $7/hour job. And they match with any slacks that aren't paired with a suit.
From my supermarket days to my service manager days, I usually wore all-black sneakers. Don't buy the cheap ones, it's your tools for work (even if you feel like a tool at work). The last thing you want to think about at work is how terrible your feet feel.
And make sure they supply you with a rubber mat. Cashiering in place for hours aches more than the average person thinks; jail cells allow more stretch-out room, to put things in perspective.