Is it necessary to pluralize the word "bag" in the sentence in the title?
"We should not use plastic bags."
(Please forgive me if it's a silly question.)
If two people go to a supermarket together and when they get to the checkout, they are asked if they would like a plastic bag, they could say "No thanks. We shouldn't use a plastic bag. We've got a backpack and a duffel bag we can put the shopping in". The person who is speaking is responding on behalf of themselves and their friend/partner. They were only offered one plastic bag so they are both turning down the offer of that one plastic bag.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.