Opinions on whether this was racist.?

I'm a 48 year old white dude and I remember similar things like that happening to me when I was a teenager. Older people often are prejudice when it comes to young teenage boys because it's always assumed that they're mischevious and untrustworthy. It could have been just because your young, but then maybe it was a combination of that and your race also. Who knows what goes through people's heads. My suggestion is just set the item on the slider next time. Grocery store cashiers don't get paid enough $ to actually pay attention and use their brains.