A good pair of noise-canceling headphones makes long flights tolerable, and a solid set should easily last you four or five years. It makes more sense to buy a durable, comfortable pair once than to replace cheap headphones every time you travel.
Audio brands refresh their main product lines every two years or so. That means last year's top model usually drops sharply in price around late November, giving you peak noise cancellation without paying launch-day prices.
Remember, a zero-interest window is a strict payoff deadline, not free money. If you still have a balance on the card after the 18 months end, regular interest applies to whatever remains.