You want earbuds that actually block out the world and stay comfortable for years. Aim for a pair with a solid build and replaceable tips so you aren't replacing them next season.
Tech brands usually drop new versions in late summer or early fall. Wait for the November sales to pick up the previous year's model, which often offers the same performance for significantly less money.
Remember that the 15-month window is a deadline, not free money. If you have a balance left when that time is up, you will start paying interest on what you owe.