Mid-September is the sweet spot for Costa Rica if you want fewer crowds and don't mind a daily downpour in exchange for lush rainforests and empty beaches. You get the green-season lull right after the heavy summer tourism clears out, which makes booking flights and jungle lodges a lot less stressful on your schedule.
A typical trip runs about $3400 once you factor in flights, places to stay, and a few excursions. Spreading that cost out takes some planning so you aren't hit with a massive bill all at once right when you get back from vacation.
Remember that the 0% intro APR for 15 months is a strict deadline, not free money. If you still have a balance left when those 15 months run out, standard interest charges will kick in on whatever is left over.