If there are only a few buyers, even if they are buying a lot, the store feels there isn't enough demand for the item among the public, and decides it isn't worth carrying.
That actually makes sense when you consider inventory control. You want to receive new product as it is needed to go on the shelf instead of paying for warehousing. It's a way to reduce overall costs. I'll do the same thing as you though and buy a lot of a flavor or brand that I like when it is there as it sometimes isn't when I want it. Two specific examples, a certain brand/flavor of potato chips and a certain wine I like. It's hit or miss if they will be there when I want them.
Another one that I kind of understand but really aggravates me. Not the store, the manufacturers. They are constantly rebranding, revising, updating things. I can kind of understand "fashion", that's such a fickle market. "Oh, that's so last month". But if I find an everyday shoe that fits and I like I might buy an extra pair or two for when that brand or style is no longer made. The one that really gets me is when they change models on something like appliances. They advertise them as new and improved. They are not. They just change them enough that you can't get replacement parts to fix it and have to buy a new one.
But I agree that postal rates have gotten out of control.
I don't blame the people at the post office for that. I consider the post office a required government service. Our economy depends on us being able to drop a check in the mail and expect it to be delivered. Basic correspondence needs to be dependable. But times have changed. We are no longer writing letters to each other, we use social media. So they don't have the income from letters or the benefits of economy of mass from those. One mail carrier delivering a lot of letters per mailbox is more efficient than one that's probably just carrying low cost advertisements.
The first postmaster general was Benjamin Franklin. Can you imagine how important they thought the post was to the success of their government if someone of that stature was tasked with setting it up?
The post office is the only federal government entity that is required to pay its own way. But they don't set the rates, Congress does. The next time you're having lunch with your US Representative or US Senator you might discuss their philosophy on why the bulk advertisers get such favorable rates. If the Post office tries to become more efficient by closing or consolidating post offices whose clientele no longer lives there (they've moved away so they have very few to serve) Congressmen go on the warpath. The post office can't do that to their constituents. I saw that when I was in Arkansas and they made a proposal to close a few really small rural post offices.
They have direct competition from FedEx and UPS for certain things. If you think the post office rates are too high, us them. A smart business would if they thought it was less expensive or more competitive.
My complain with the post office is not the rates. We live in a changing world, what used to work doesn't. My complaint with the post office is that it is no longer highly dependable. Stuff just isn't being delivered with the surety it used to be. I noticed that about the middle of the previous administration when the top people at the post office appointed by that administration started trying to run it more like a business than a government service. I don't like government wasting money. Times have changed and continue to change. The post office has to get more efficient, in cost and dependability.
I don't feel they've kept the priority that it is a necessary government service, just purely looking at the money. With all this talk of privatizing, I wonder how that would work for rural America where it's not as cost effective to provide a service. Big cities are easy, small town America is not. Or what rates they would charge to serve remote areas?
A recent story. When my parents died my brother and niece collected their photos and sent them to me. I scanned them and put them on a flash drive. I mailed six flash drives to five different states, using the least expensive method but so I could get tracking. No priority, just basic mailing. Two of those six packages arrived the day before the estimated delivery. One was about a week late, another 2-1/2 weeks late. The last two were over two months late. That's what I'm talking about as far as dependability. That's the dramatic change I've noticed in the last four years. This used to be rare, now it's common.
I don't want to waste money but if I want a service I am willing to pay for it. Raising the rates enough to get that service without using tax money to subsidize the post office is going to hurt a lot of people. Raising taxes will hurt people. This is one of the many things I don't have a good answer for.