The reusable bag does indeed provide the benefits you describe. Every time the shopper uses it, the grocer does not have to supply paper or plastic bags. The latter cost about 1.3 cents each, at least in boxes of 1000 from Sam's Club. Maybe 1 cent each if purchased in even larger quanties by a big grocery store. A reusable bag should, therefore, easily pay for itself in half a year. Few investments have payback times of half a year!

On the other hand, if the grocer therefore pays less for the materials and energy needed to produce the non-reusable bags, this needs to be reflected either in the grocer's prices, or else in a micro-discount for using the returnable bag. Otherwise, the customer is paying to enrich the grocer at his own expense. Plenty of schemes are available to share the benefit. E.g. you can buy a 99 cent reusable bag from Target and, every time you use it, you get 5 cents off your order; it pays for itself in 20 shopping trips. A vitamin store once sold a reusable bag for $5.00, and then gave a $1.00 discount on purchases over $10 every time the bag was used. Both systems followed Henry Ford's principle that all parties to a transaction must be better off for the transaction to be viable.

The bottom line is, however, that if the grocer in question was really, really concerned about the environment, it would give out the returnable bags for free--maybe one for every few hundred dollars of purchases, as tracked on a shopper's club card, so people wouldn't grab handfuls of them for non-shopping related uses--instead of selling them.