Since you'd be giving that customer 5-10 fewer reasons to come into your store over the next 10 years, I'd say you'd have the potential to lose plenty of $$$ beyond the .014 allocation cost of each 0-ring.
But beyond that, there's the wonderfully naive notion that a customer should expect to be charged the same for an item as what the business paid to acquire the item. By definition the selling cost of an item needs to be set higher - often much higher - than a retailer's acquisition cost. (There's that pesky "economics" thing again.) In your world you seem to think that the retailer should only charge .014 for an o-ring that cost them .014. (Actually you seem to believe that they should charge $1.00 for $1.40 worth of o-rings, but that's a different issue.) In the real world, determining the actual cost and realistic selling price for anything and everything needs to consider a lot more.
So, no, putting 80cf of compressed air may not specifically cost the retailer $5.00. But let's look at all the things that go into why it may cost YOU $5.00...
What a dive shop needs to factor into their actual cost for - and realistic selling price of - providing an air fill:
- Direct compressor use cost: total cost to power, run, maintain, inspect, service, and repair the compressor (including consumables and parts) for a year, divided by the total number of tanks filled per year = $X/tank fill
- Direct compressor amortization cost: total purchase price of compressor divided by the working life of the compressor, in tank fills = $Y/per tank fill
- Direct labor cost: total annual cost of an employee (wages + bonus + benefits + employment taxes + training + all other employment costs) multiplied by the percentage of an employees time spent on all activities associated with filling tanks over the course of a year (including compressor maintenance and repair) divided by the number of tanks filled in a year = $Z/per tank fill
- Allocated overhead: Total annual business costs for things such as rent, utilities, fees, insurance, training, memberships, etc divided by the proportion of the business comprised by tank fills per year, divided by the number of tanks filled per year = $A/per tank fill
- Allocated SG&A costs: Total selling, general, and administrative costs associated with running the business for a year (including such things as pool heating/service, accounting and legal fees, advertising costs, web development and maintenance, trash services, plowing snow from the parking lot, cleaning services, spoilage, shringkage, etc) divided by the proportion of the business comprised by tank fills per year, divided by the number of tanks filled per year = $B/per tank fill
- Indirect/Allocated labor cost: Total annual cost of all employee time (wages + bonus + benefits + employment taxes + training + all other employment costs for every employee in the organization) that are NOT BILLABLE and/or that do NOT RESULT IN A SALE, divided by the proportion of the business comprised by tank fills per year, divided by the number of tanks filled per year = $C/per tank fill
- Unallocated Inventory Carrying Costs: the total cost - net of direct acquisition cost - associated with purchasing, receiving, stocking, and distributing all items held in inventory over the course of the year, divided by the proportion of the business comprised by tank fills per year, divided by the number of tanks filled per year = $D/per tank fill
- Allocated and Unallocated debt service costs: total interest and other debt costs incurred by the business in the course of the year divided by the proportion of the business comprised by tank fills per year, divided by the number of tanks filled per year = $E/per tank fill
- Opportunity com: the value that the time, money, and resources put towards filling a tank if they had been applied to another activity = $F/per tank fill
- Net Profit: The desired/required return in ex change for delivery of the specific goods/services, after taxes = $G/per tank fill
Now, I have no idea what the specific numbers would be for any of those variables for any given tank fill or o-ring at any given shop. But with the realization that they are all positive, real numbers - that represent actual costs that any and every business incurs - it's not hard to imagine that $X + $Y + $Z + $A + $B + $C + $D + $E + $F + $G could reasonably cost the business more far more than $5.00 to provide YOU the tank fill.
At least for a business that is going to STAY in business.
Now, I'm just using o-rings and tank fills as an example. But the cost allocation above need to happen for every single item and service that your LDS provides. So all you people that want things for free, and that don't want to pay more than the shop's acquisition cost for an item, and want cheap airfills, and have the store open at convenient times for you, etc... and everyone in the "If I owned a dive shop" contingent who would have three of everything, in every size, in stock for people to try for free in a heated Olympic swimming pool, and have a fun hangout place, and free beer, with knowledgeable, GUE-trained, tech-diving supermodel sales people, and leprechauns staffing a unicorn petting zoo for customers' children... you really do need to consider the economic realities of running a business in the real world.
There's just something about being a retail customer for hobby/pleasure-related goods and services that cause people's brains to disengage when they have to put their hand in their wallet. Add to that an insanely presumptuous sense of entitlement by many. We see it here. You'll hear it if you talk to cyclists, or skiers, or boaters, or hunters, and probably if you talked to knitters. But I've never heard anyone ever question how much Safeway paid for the can of beans that they just forked over $1.29 to buy... or whether it "really" costs the barber $20 to cut their hair, or how much the beer in their $3 draft actually cost the owner of their favorite bar.
Would a sane person ever suggest that a local restaurant should give you them french fry - which only cost the restaurant $0.014 - for free whenever they stop by and ask for one?