Why can't I pay you?

Imagine walking into a shop and buying 3 apples, except after paying you realise actually needed another apple. You go back in and pick one more apple from the pile, and go to the checkout... and the person behind the counter refuses to sell you another apple.

"You've already bought 3 apples" they insist, "you'll need to return those 3 first and then buy four. You can't just buy one more!"

It's a quirk, you think - probably some dumb computer system somewhere. You offer to return the apples so you can buy 4, and they helpfully point out that the apples are non-refundable. So, you actually can't return them; you can throw them away and buy four apples at once, or you can stick with the three you have.

Obviously that's ridiculous. The shop's business model is to sell you things and you want to buy things from the shop and they have plenty of apples so this should be simple! Of course, in shops which sell apples this wouldn't happen. They'll gladly sell you another apple because that's what they do.

Except

I recently went onto a travel website to book a hotel. I found a good deal which I was happy with and I booked Sunday to Wednesday. I went to sleep happy with my choice.

The next day, I woke up to realise I actually should've booked Sunday to Thursday. I go onto the travel website to extend my stay and I'm told that to do that I would need to cancel and rebook. There's a no-refund cancellation policy so I'd lose all the money from my previous booking.

I book a room in a different hotel using a different travel website.

Now, fair enough, the original booking was non-refundable. But I didn't ask for a refund - I asked if I could buy more of their thing. I wanted another night! I would've moved rooms if I'd had to. But no; it wasn't at all possible to get another night added on. It's absurd, and I'm pretty sure that while there's probably some technical explanation of why things had to be this way, that doesn't do me any good, and the hotel - and the travel website - lost out on the chance for me to pay them money.

But Also...

I run Pi-hole on my home network, and sometimes the DNS blocking breaks things. Usually it'll be a website that goes wrong in some way. I'm reasonably good at spotting this, and I don't mind the trade-off. It works for me.

What I can't understand is why any online retailer would be OK with any part of their payment page being blocked. Multiple times, I've been unable to pay for a product online only to have it work if I switch my DNS servers.

How can your business's tracking or advertising be more important to you than taking my money? The whole point of your store was to take my money, and I wanted to give it to you!

Again, probably there's a technical explanation. Maybe they don't mean for me to be blocked. Maybe they'd change it if they knew. This is just a rant, but it doesn't matter; why can't I pay you?