Shadow work is a term used in economics to refer to work that was once done by companies, but has now been outsourced to customers, generally in the form of self-service operations.
This is by no means a new thing. Elevator operators, bellhops, telephone operators, and milkmen all used to do useful work for us, and now we are stuck doing these things for ourselves. More modern examples include self-service gas stations, the replacement of bank tellers with ATMs, and self-checkout and bagging at grocery stores.
The term was coined in 1981 by Austrian philosopher Ivan Illich, who felt this was a bad thing. There is some debate about this. It is not obviously the case that paying someone to bag your groceries for you is a good default; the success of stores such as Aldi and Lidl indicate that many people feel that it is not worth paying for. Likewise, full service gas stations did not disappear because no one was willing or able to provide gas jockeys, but because customers just didn't care enough.
Moreover, there is a strong case to be made that shadow work is not a unidirectional force. While Walmart is happily providing an option for self-checkout, they are also providing in-store pick up and delivery services in many areas -- indeed Walmart has decided that part of its services to the customer should include electronic shopping trolleys for those of limited mobility, which is very much a new service in the retail landscape. Likewise, most retailers have taken credit card processing fees upon themselves as a matter of convenience to their customers; many of us recall when credit cards were not universally accepted, and where they were there were often additional fees attached to their use.
Most shadow work is due to technological advancement; in some cases, such as ATMs and elevators, technology has made some tasks so easy that it is no great burden for most people to perform them; in other cases, such as gas-stations and grocery stores, security has improved to the point where companies are no longer quite so concerned with customers walking off without paying. In some cases jobs just became too hard; it is simply not efficient to have an operator look up telephone numbers for customers once the network gets too big. While it is not necessarily the case that advances in technology will make up for an increase in shadow work, it is, as always, the case that customers will get what the average customer wants. This means that some of us will be upset that companies ask us to bag our own purchases, and some of us will be upset that companies do not penalize those morons who leave their carts in the middle of the parking lot. C'est la vie.