Clarins Skin Beauty Repair Concentrate - I love face oils. Period. And I love all of Clarins' Facial Treatment Oils. Santal is the one I use when my skin is acting irritated, but it doesn't calm my skin down as fast as the Skin Beauty Repair Concentrate. Which is annoying because this is a tinier bottle with a bigger price tag. What's more annoying is I don't actually know why it works as well as it does - but I always have a bottle on hand for emergencies.
Lash Serums (Revitalash, Lilash etc) - Lash growth serums - the proper kind with bimatoprost derivatives in the formula (as opposed to the majority of the stuff you buy off the counter which is just glorified hair conditioner) - is to me what Botox is to some people. You don't need it, but once you try it you have a fear of being without it. Which is a pain because it's not cheap and you need to keep using it for your lashes to stay long. Just as I would say of botox, if you're not able to afford it for the longer term, you're probably better off not starting.
Benefit Gimme Brow - I feel a twinge every time I need to buy a tube because, dang it, this tube is tiny for the price. But I've tried so many brow gels (including ABH) and none can hold my brows in exactly the angle and position I want them, while making them look thicker, while being the perfect ashy brown tone all at the same time. So I wince, and I pay.
(And if you're curious why my staple Shu Uemura Hard Formula 9 pencil isn't here, that's cos I feel no twinge when I buy one of those - each tube lasts up to a year when I get them shaved sword-style, so I find them far cheaper than lower-priced pencils like MAC's which I used to love but would run through in 2-3 months.)
I'm sure I'll think of half a dozen more later. What are some of the luxury beauty items you love but that you also know you probably don't need?