Correct use of the comma.

Alright, Southperry is full of literate people, like Aaron, rite?
So here's a srs grammar question that I always contemplate against when listing items with commas.

Here are two sentences to give an example:

[SIZE="3"]I drove down to the drug store and bought milk, a magazine and gum.

I drove down to the drug store and bought milk, a magazine, and gum.[/SIZE]

Which one is considered grammatically correct?

Now, for years I've always seperated the last item with a comma. Last night, my mom and dad were reading over an essay that I left on the counter to take to school tomorrow. They teased me for making a grammar error on the second sentence, and I couldn't believe I missed it. I took the paper and read the sentence over and over and couldn't find anything wrong. Both of my parents agreed that the last item, when listing with commas, should NOT be seperated with a comma. Apparently, this is how they were taught in school back in the day. I asked my english teacher while I was at school today and she couldn't give me a definite answer.