I was trying to figure out the same here, but so far I didn't have any luck. For example, the "Wasteland 3" and "Wasteland 2: Director's Cut" was in sale and I put it on the cart, but two days after it, I got the message that the price of added items to the cart was been changed since the last time, so I checked the games store page and it looked like the sale ended for both of them, but 24 hours later, while searching for games, I found that they are discounted again, like if they are on sale again or maybe their sale never ended at all.
So indeed it kinda sucks, the game sale not having a expiration timer like it used to have, but taking in account what I experienced, maybe it's a bug of sorts? Since they uptaded the cart and other features very recently. Peharps we should report this whole situation as a bug, with support tickets

:(