I’ve noticed this with grocery shopping online lately too. I’ve booked a shop, checked it out and then when I’ve returned to it to make changes, there are alerts to say things I’ve added days ago are out of stock, and suggesting alternatives. And yet on certain items your choice seems to be firm, as you can’t alter the quantities after a certain time. Tesco seem to do this a lot. I’ve been after a speciality gin from them for a while now, and every week they let you add it to the basket and then substitute something else on delivery.
The Next online sales irritate me too. They give you a preview and then make you book a time to shop the sale. You get access at your allotted time, make your selection, you’re allowed to add it to your basket and then when you get to the checkout you find they’ve sold out. So frustrating.