A good mechanical keyboard should last you a decade. You are paying for heavy aluminum cases, hot-swappable switches, and keycaps that won't turn shiny after a few months of typing. Buying right the first time keeps you from burning money on cheap plastic boards you will replace in a year.
Direct-to-consumer group buys often end up with extra inventory after fulfilling orders. These extra sets hit vendor sites as overstock sales at flat retail prices. You get custom-level quality immediately without waiting a year for a manufacturing run, making last season's overstocks the smartest buy on the market.
That 15-month window is a strict deadline, not free money. If you carry a balance past month 15, the standard interest rate kicks in on whatever balance remains, making your desk upgrade much more expensive than planned.