Nothing for you to feel bad about, they made a mistake. I’m sure they preferred this solution to having you return the bottles. This kind of minor product loss at grocery stores is common and not worth their time putting any more effort into. Hopefully nobody decided to push any blame down the line to some poor delivery guy.
As long as we’ve expanded the discussion to include seemingly weird pricing:
I buy Iams cat food at my local Safeway for my three cats. The bags come in two sizes. The usual rule is that buying in bulk is cheaper, but I always check the per-unit price. Weirdly, the small bags are $2.79/lb and the large bags are $3.15/lb.
I re-check every time I make a purchase because I can’t figure out why they do it. But it’s great for me, as buying more smaller bags means the food is on average fresher than it would be if I were less frequently opening large bags. And we all know what picky bastards cats can be.
They do it because most people don’t check and so they sell most of the volume of product at the higher per-pound price. Easy peasy.
For some low-value products it’s certainly possible that with modern logistics, more of the grocer’s cost is per the package, not per the goods. I.e. a pallet with 500# of cat food is cheaper & easier to handle at the store if it’s 10 50# bags instead of 100 5# bags. If labor is expensive enough and cat kibble is cheap enough, that could create the price inversion you see by them honestly applying the same markup to both products’ total cost of goods sold.
Color me skeptical it’s really that though. I think it’s just a trick play that works on the rubes.
But in that case, wouldn’t the “bulk is cheaper” rule apply - the 50lb bags are easier to handle (require less labor, that is) than the 5lb bags, so the per-pound cost should be less in the larger bags?
I suspect your other explanation is the correct one, however.
I think there’s a diabolical science behind it all. Maybe the cat food thing goes like this… Cats are picky, so people routinely buy things that cats reject. So let’s set a low price point that allows people to try it. If the cat likes it, owners will be happy to buy it in larger quantity. They won’t check or notice, as @CairoCarol did, that in fact they’re paying more.
Sometimes it’s the mere power of suggestion. Put the word “SALE” in front of it and some are going to assume it’s a good deal. “20% off!” may be off a price that was way too high to begin with. Everything in the store costs a dollar, but sometimes a dollar is more than you should pay for it. And I notice that sometimes bulk is a good deal, sometimes not, when you calculate it out.
Bro told me he read about this Wal Mart trick years ago. Take a product…say, a 2 slice toaster. Wal Mart may offer half a dozen of them. They price the cheapest one very low. Customers see that price, realize it’s good, but they know that cheapo items may not last. They relax and buy something higher up the chain, erroneously assuming that the prices are still competitive based on the lowest rung.
Another one from years ago: shrinkage. Ask George Costanza:
Wow, that’s rather wonderful. 
Extra work for the staff, though, who are likely to be busy, overworked, and underpaid already, but it does sound pretty good.
I have usually found substitutions perfectly acceptable, although with the odd occasion of wanting instant decaff for my Dad and receiving coffee beans instead.
I think people tend to tip generously because products are cheaper when ordered online. Employers have found ways to make the staff behave nicely such as the possibility that clients should rate the personnel or tip them by card.
I’ve used Shipt here in the US and for each item on your order you can separately choose “no substitution, just skip it”, “call/txt me to discuss any substitution”, or “subsititute as best you can”.
That seems to be a typical feature of other delivery services too.
At least IME, the prices are higher online.
Yes in general places like Amazon are cheaper than traditional bricks and mortar retail.
But for delivered groceries or other local retail using either a 3rd party delivery service or one the store has contracted directly with, e.g. Shipt as cited above, there is a large and unstated mark-up on the online prices over the in-store prices. Then on top of the hidden mark-up they also charge a monthly subscription fee, and they guilt you into tipping the driver (who’s truly underpaid).
Overall after a month’s experiments with Shipt I found they’d tacked 30% onto the cost of going to nearby retail stores and had barely half the selection vs. what’s on the shelf. All while their website loudly touted that there was no delivery charge! I cancelled my trial subscription and gave their survey a scathing rating (not that I expect that that accomplished anything.)
I had a weird substitution at Sainsbury’s (UK) a couple weeks ago. I ordered some Polish snack sausages (think Pepperami or Slim Jim) and instead received a tin of slid (fish) in tomato sauce. The only connection between the two is that they were both Polish and the same price.
Wow, Romania has sure changed! When I was there in 1990, you often couldn’t even find an open restaurant for dinner or a store with much of anything on the shelves, let alone have groceries delivered.
Yes, it’s different. Although most people were hoping there were going to be more drastic changes.
But the 1990s were bad, I remember.
ETA: In 1993 inflation was 300%, for example.
I was there with a contractor doing a project for the US embassy. We wandered the streets looking for an open restaurant one evening, finally finding one at a hotel. It was empty and we ended up having one of those circular conversations:
What do you like?
What do you have?
We have everything.
Steak?
No steak.
Fish?
No fish.
Well, what do you have?
We have everything. What you want?
Chicken?
No chicken.
We finally just left.
Anyway, back to the Great Diet Coke Calamity.
Over last weekend, I discovered my farm store in town was closed for the holiday when I went to buy chicken feed. I looked on line, and saw that Amazon carries 40# bags at prices between $50-$65. 
They are all rated at 4.5-5 stars, indicating people are happy with that. I pay around $15/bag at my feed store.
Never mind.
It’s that daft Monty Python cheese sketch all over again.
Should have asked if they had Venezuelan beaver.
I’m not familiar with that, but it sure seemed like we were in a “Candid Camera” scene.
You know the old economics lessons that the higher the demand, the higher the price? Somewhere I read that Amazon puts that philosophy into its pricing algorithms, and you don’t even have to buy the product to have an effect. The more people look at a product, the higher the price goes.
For example, a few months ago, I lucked in to a case of 4 1-gallon bottles of Simple Green D Pro 5, which is a strong disinfectant that is effective against viruses, for about $65 for the case, from a 3rd party seller. [I gave one to my BFF and one to my neighbours, who have been tremendously good for me over the years. It’s concentrated, so 1 gallon makes 64 gallons of cleaning fluid. The 2 gallons I kept will last a *long* time.]
At the same time, Amazon was selling a single gallon for a bit more than $65. I reported them to themselves for price gouging, which amused me no end. It’s been 7 months since then, and you still can’t find a gallon for less than $70 on Amazon. (Sears, oddly enough, appears to be selling it for $30/gallon.)
Not necessarily, Amazon changes prices all the time.
Here’s what happened when I tried ordering some wasabi peanuts from Walmart:
I got a refund for the dog biscuits and donated them to the local animal shelter.