You are getting this soundbar and subwoofer to turn your living room into a proper theater that handles movie nights and game days for the next five to seven years. Buying once means skipping the cheap bar you will replace in eighteen months.
Wait for the Super Bowl sales in late January when retailers slash prices on last year's gear to clear shelf space for new spring inventory. Last year's model usually has the exact same audio quality as the new release for a fraction of the cost.
That intro window is a hard deadline, not free money. If you still have a balance left when the 15 months are up, interest charges will kick in on what you owe.