I've have the same issue as maratd where on the product page it has said:
* Prime
* "Want it $DayOfWeek, $Month $Day? Order within XX hrs XX mins and choose..."
where "$DayOfWeek, $Month $Day" is 2 days away and I've bought it and it didn't show up until 4,5,6,7 days later. Same thing with the email. It's not "sub-par UI" it's very clearly designed to make you think... no fuck that, it's not just "thinking" they TELL YOU when you will get it and even give you a countdown to "lock in". It's not even deserving of "shitty UI" if they can't honor that text they shouldn't show it. Period. You can't lie on your product page then turn around and point to emails no one reads confirming my purchase. I buy prime because I know it will get here within 2 days and if I can wait (which I normally can) then waiting is worth not having to deal with more shit traffic and going to the store. If it's not going to be here in 2 days I need that in FLASHING RED LETTERS, sometimes I will still be fine. Sometimes, like when it's medicine, I need to know it's not coming when I expect it.
I've had times I've ordered something with the text from above, it not showed up after 2 days and then Amazon telling me it won't be here for another 3-4 days when ON THEIR WEBSITE it still say's "Want it $nowPlus2Days? Order within...". That's bullshit.
All of that said I love Amazon and this inconvenience is minor compared to the value they provide. I just wish they would be a little more honest on the product page. They KNOW this is happening but are choosing to do nothing about it, that really disappoints me. I expected better from Amazon.