You spend a third of your life at your desk, so a good mesh chair with real lumbar support is about saving your lower back from daily misery. A proper one should last you a decade, which means you buy it once and stop wasting money on cheap desk chairs that sag after six months.
January is prime time for work-from-home furniture promotions as retailers clear out inventory. Look for last year's model instead of the brand new release, because office chair updates are mostly minor cosmetic tweaks while the ergonomics stay the same.
That 18-month window is a hard deadline, not free money, and if any balance is left on the card when it ends, you will get hit with retroactive interest on the original amount.