[shabble]

If there would be a realistic way to reliably identify 'luxury' purchases, you could just have a really infuriating interstitial that:

* displayed the amount of money in a personally relatable manner - "ShineyTron300 costs the same amount as your last 2 months of food expenses, are you sure? (y/N)

* make it easy to cancel/stash or defer the purchase, including as you suggest, an enforced time delay before re-prompting

* make it hard to complete (require several captchas, solve a maths puzzle, or complete a couple of levels of Duolingo or something) before letting you through.

* Enforcing a personal 'luxury sales tax' that padded the total amount of some %, and transferred that amount to savings if you complete teh purchase.

Basically, go hunting for dark patterns, and use them against yourself.

I'm not sure how far you could go without it just being too annoying and disabling/bypassing it though.