>>610772
I'll offer a semi-educated answer.
You're a part of the global value chain that stretches from the labor that acquired the raw materials, to the factory, to the truck, and finally to the store. You're at the end point.
Without your labor things aren't sold: you're the guy who puts things on shelves, points people towards the specials, and takes their money, as well as doing a whole bunch of other shit that ensures the store runs efficiently.
So, yes, your labor is "non-productive" in the sense you aren't creating commodities, but you are ensuring they are sold which is a valuable service to the bourgeoisie, hence your position.
As to how much exactly? Dunno, only the boardroom might be able to give you a definite answer (if they cared about you individually). Lets say for simplicity's sake you and all your co-workers in all the stores in your chain are worth 25%, the other 75% going to the rest of the value chain.
Now, the company, in all likelihood, does not directly own and run all the companies that supply it as subsidiaries. But that surplus value generated from the rest of the chain still arrives in the store, and is then expanded again through your labor. Large chain stores, for example, are notorious for ripping off their suppliers. It buttresses their profit margins; it's exploitation on top of exploitation, all down the line.
Each company that supplies another has to have some kind of regular profit to survive and make the whole thing worthwhile, hence surplus labor is being extracted in every business that interacts to put a commodity on the shelves.
To wind things up, it affects you because every laborer before you has been exploited in order to squeeze a profit out of the operations that stocked your store: you have to be squeezed in turn to ensure that surplus value does not end up in your pocket undeservedly – it's not yours, it belongs to those who own the store, who argue that without their existence the value chain would not have been created in the first place.
This is what neoclassical theory has helped me understand: laborers get what they're worth, i.e. what they individually contribute to the value chain from dirt to store shelf. Porky, situated on top of his private property, hires others to make it productive and thus gives them a wage in return. As porky has the highest marginal productivity, by virtue of the property and technology owned by him, it is only natural he profits.