A good pair of noise-canceling earbuds is an investment in your sanity on long flights. Buy a top-tier pair once so you do not have to replace them every year.
Wait for the next flagship model to drop before you buy. When the new version arrives, the current model usually gets a price cut, giving you the same premium tech for less money.
That 18-month window is a deadline, not free money. If you still have a balance once those 18 months are up, the bank will charge you interest on what is left.