You want earbuds that actually block out the world and last for years, not months. Buying quality hardware once saves you from having to replace cheap, broken plastic every season.
Wait for the tech giants to drop their newest models. When the hype dies down, last year’s version usually gets bundled with phone accessories at a discount. That is your window to buy.
Remember that the 18-month window is a deadline, not a gift. If you still have a balance when that time is up, you will start paying interest on whatever remains.