For Many Retailers, the Price Isn’t Right, The Wall Street Journal. How important are prices in the battle of brick and mortar stores against internet competitors? Are there other considerations that may be more important? If these companies are striving to maximize their profits, does it make sense to ignore or reduce in its importance the pricing concept of MR=MC?

The issue of price is vitally important for "brick and mortar" stores when considering the competition from online retailers. In fact, the gradual but steady disappearance of brick and mortar retail book stores is a direct result of the competition posed by Amazon.com, which offered books at discounted prices and, for many years, was not required to charge customers state sales taxes. The combination of those two factors--discounted prices and no state sales tax--enabled Amazon.com to dominate book sales while driving many brick and mortar book stores, including once-veritable chain Borders Books, out of business. To survive, Barnes and Noble, the other main book store chain, had to similarly develop an online purchasing option. In addition, Barnes and Noble was forced to develop its own electronic reader (e-reader) in order to compete with Amazon's Kindle. So, as you can see, the competition from online retailers had a very serious impact on brick and mortar businesses. And, the success of Amazon.com forced many other types of businesses, including clothing retailers and auto parts stores to follow its lead and develop online purchasing options.
Cost was an important component of the emergence of competition between brick and mortar stores and online retailers. The success of most states in compelling online retailers, again, with Amazon.com as the leading example, to charge customers sales taxes consistent with the tax rates of each individual state reduced the online retailer’s price advantage over brick and mortar stores, but the damage has been done.
Price is not the only factor that determines the consumer’s choice between online and brick and mortar store shopping. Many consumers prefer to physically walk into a retailer and browse items, try them on in the case of clothes or thumb-through books in the case of book stores. The option of purchasing right-away rather than waiting one to four days for a shipment to arrive remains an important consideration. Similarly, it is far more convenient to purchase auto parts at a neighborhood store than to shop-for and purchase items online in the vast majority of cases, as the parts are generally needed right away and shipping from a distant warehouse obviously takes more time. The importance of price in influencing consumer decisions, however, remains exceptionally relevant.
With respect to the MR=MC rule, in which the cost of producing an additional item should increase profits for the manufacturer, this is a basic consideration for all businesses, and it involves myriad considerations, including the reliability and costs of material supplies needed for the manufacture of a finished good. It also involves calculations of future demands for a specific good, all of which requires studious attention to changes in or evolutions of consumer tastes and needs. Producing extra goods because the profit margin appears favorable relative to the costs of production is not necessarily a good idea if market conditions change, thereby rendering surplus goods obsolete.
MR=MC is more about manufacturing processes than about the consumer retail business. In a sense, this is confusing wholesale and retail industries. Suffice to say that price is always a consideration. Even individuals who prefer to purchase books in brick and mortar stores will usually opt for the cheaper online option if the price differential is significant. Additionally, prior to the success of the publishing industry in requiring Amazon.com to price e-books at a rate commensurate with that industry’s demands for a more equitable distribution of profits, the lower cost of e-books was driving consumers away from hard-copies of books towards electronic versions of books. The publishing industry’s success in forcing online book retailers to pay publishers more than had heretofore been the case drove up the costs of e-books and made them less competitive with hard-copies available both online and in those brick and mortar book stores that have survived.
https://consumerist.com/2015/09/04/e-book-prices-increase-sales-slump-after-amazon-publishers-finalize-contracts/