Stores are not doing me a favor by giving me a method to give them money. If they can't financially justify a night time cashier, nothing is saying they have to be open all night. Presumably they do so because it's profitable for them. I'm sure none of us really believe that our convenience is the real reason they are open 24 hours?
I think it's really bad business to expect to take the same amount of money per item at 1am that you do at 3pm but refuse to offer even the most minimal service in return. A cashier is a basic, minimal level of service. The Denney's example is actually a good one....no matter how awesome and convenient it might be for me to procure a Moons Over My Hammy at 2am, I still expect a server to come and take my order and bring me my food if they are advertising themselves as a 24 hour full service restaurant. Don't send all the servers home and tell me you are doing me a convenience by allowing me to wait on myself for the same menu price you charge during the day. Either the store is open all night, or it isn't. I can totally get behind the fact that at 1am there may be a slight wait for a cashier to come up and open a lane, or that there may only be 1 lane open instead of 10. But expecting ONE employee capable of helping customers at a 24 hour store is perfectly reasonable and not at all special snowflakey.
Mark is employed in the middle of the night for a reason, and I'm sure it's not so he can chat with another employee while a customer, who has already asked for help, struggles alone with a system not designed for large carts of food. We all know that an employee has the ability to override the scale so that at the very least the customer can move already checked items back into the cart. Mark didn't even do this very easy, very basic thing to help. Sounds to me like he just didn't want to bother. A letter to management is definitely justified. I'd be shocked if management replies that Mark was just following policy by refusing to help the OP at all.