Tiered Spender promotions (prizes stack) and Bonus Ladder promotions (prizes do not stack) are different. Keep in mind we will probably getting alternating types of promotions in the future. Keep an eye out. It should be noted that any additional comments regarding feedback of the promotions or any Item Mall content should be addressed here
Indeed, I just wanted to verify that you understood the promotion lasted more than a day. This will not be the last time we will see this item, I highly expect. So if you can't do it now, perhaps there will be a future promotion where you can be able to purchase it.
Not sure if it interests you, but keep in mind for 8k AP, there are the 15 loot boxes, and for 10k AP there is a 50/50 chance for a Seastar outfit. So even if you do not spend 15k AP, you can always go for a lower step of the ladder.
I suggest voicing your opinion on the official feedback thread here