1) The avoidance of the negative consequences for not doing it. If someone 'catches' you not doing this, your reputation is harmed. You are acting out of the self-interest of protection of reputation. And the same 'rule of thumb' applies when nobody is watching too.
There are no negative consequences.
Maybe supermarkets in the UK are different. Here in the US, one is not expected to push the cart back to the store. There are store employees who do that.
2) By pushing grocery carts back to the store, you are instantiating the meme 'push carts back' which increases the meme's chance of being replicated by observers (or even those that merely observe the consequences)
Oh, so it is all a matter of memes. One presumes that these memes are made of an immaterial spiritual substance, part of a dualist account.
Still, memes are not motives. I don't see how this is responsive.
3) By helping your community, you are keeping the community strong. Since your community (From the gene's point of view) is probably composed of those closely related, this helps (but not much, obviously) the chances of the gene that promotes community helpfulness to replicate.
It's not much help to the community, since store employees will eventually collect those carts anyway.
Jesus was a liberal hippie