A car ignition that stops turning is one of those repairs that cannot wait. It keeps you from starting the engine and leaves you stranded, so you need a fix immediately before the cylinder jams completely. Mechanics usually charge about $350 for this job, which covers the part and the labor.
When you head to the shop, ask for an itemized quote so you know exactly what the labor costs versus the hardware. Since this is an emergency repair, you likely won't have time to wait for seasonal discounts, but you can manage the bill by spreading it out rather than paying it all at once.
Remember that the 0% window is a deadline, not a gift. If you have any balance left after 18 months, the bank will start charging interest on the remaining amount, which can get expensive fast.