It is faster to ring and then bag as each item must be put in the back differently and many don't feeds the belt correctly (they put the stuff on top first which is usually the light, smushable things, like bread, eggs, raw meat, fruit, etc.) So rather than sort and bag each item 1 item at a time and bag later (assuming to provide bags) it is done afterwards. Especially, for someone like me who uses a backpack and 2 reusable bags, it is easier for me to bag it myself and put the weight where I want it. Additionally, they don't have dedicated baggers like they did 20 years ago. I guess pay more for the service and product price increase to have it bagged for the added cost of the bagger's $15/hr x the number of employees x 6750 hours per year x 162 stores = just shy of $48 million at an average of 3 baggers.
I am not sure why that is being done. It is time consuming. I have gone plenty of times most of the cashiers I have had put them in the bags after ringing things up. But I did have one cashier ask after ringing everything up if I wanted to bag my own groceries. I guess it would depend on who you get.
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