Your back shouldn't hurt by 2 PM. A high-end chair changes how you feel after an eight-hour shift, making the workday feel shorter and less draining. It is a tool for your body, not just a piece of furniture that looks good in a room.
Most high-end brands run their deepest discounts during Labor Day or Black Friday. Before you commit, visit a local showroom to sit in at least three different models. Comfort is subjective, and you don't want to spend hundreds on something that doesn't fit your frame.
The 18-month window is a firm deadline to pay off the balance without interest. If you still owe money after that time, the standard variable APR will apply to whatever balance is left on the account.