A good retirement BBQ is about the people and the grill, not the stress of a big bill on Monday morning. You need time to watch for sales on brisket or ribs and to get the guest list right. Starting a few months early lets you buy the non-perishables when they are cheap rather than paying full price the day before the party.
If we spread the $700 cost over a year and a half, the math gets much easier. It turns a big one-time hit into a small monthly habit while you focus on the guest of honor. We can make this work by using time to our advantage.
This plan only works if you pay off the balance before the 18 months are up. If you still owe money when the clock runs out, the interest kicks in on whatever is left, making those burgers a lot more expensive than you planned.