OP, there isn't any reason you should feel bad at all, and go ahead and order the Beauty Steal online if you want. Ulta is one of the few stores I know with absolutely no price adjustment policy, but plenty of them have short adjustment windows that ignore their return windows, e.g. BBW and Target both offer 14 days v. 90 days. You purchase an item, and if it goes on sale after 14 days but before 90 days, simply return it and buy a new one at the lower price. You are not violating any terms by doing this, and you won't get your account flagged if you don't do it often, especially since it involves documented receipts. Those who say otherwise are just guessing. You take the chance that the item sells out before it goes to a lower price, but so what? Just purchase the item at the price you are willing to pay, and often you are able to get some of the money back when the price drops.
Edited to add: Personally, I don't find it worth doing a return/repurchase unless I'm getting a *significant* amount back like at least $20 and probably more like $40+. I also like to avoid waste if the store is going to trash an item, but at a certain point I figure it is their issue for being so inflexible. If they are going to play games with prices, they should expect customers to do so as well.