overtime in a grocery store is rare?

Usually over time is a benefit that full time workers get. It's not common for retail or grocery store staff to be full time. He can ask you to work extra hours (if you're part time you don't get over time and nothing changes doesn't matter how many hours you work), but you have the right to refuse if you can't work the extra hours.
If you're full time and you have overtime as a benefit, then you should talk to your boss if you're going to get over time pay if they ask you to work extra hours. If you refuse, they will move onto the next available person that can / wants to work the extra hours. You didn't tell us where you're located. I'm guessing some where in Canada (I live in Ontario) since IGA is a Canadian grocery store.
Usually grocery store workers are payed hourly it's not a salaried position, which they're not going to double. Usually with over time you get time and a half for the extra hours you worked.
I work full time about 40-45 hrs a week, but not for retail or a grocery store.

That is up to you.

You have to decide if the extra money is worth the extra time worked to you.
You may be entitled to overtime pay depending on where you live.

accept!!! $$$$$$$$$$$$$$$$$$$$$$

That overtime pay adds up to a very nice chunk of change. Actually, overtime for grocery stockers is fairly common, and rather lucrative. It's also commonly the first job in the training chain to move up within the organization. If you refuse, you're an idjit.

You either want the extra money or you want the time off. Pick one.