Costa Rica in the shoulder season means misty mornings, empty trails, and way lower prices on everything from canopy tours to eco-lodges. Going between May and November brings afternoon rain, but you get the rainforests mostly to yourself and save a bundle on flights and hotels. It is the best time to see the country if you do not mind carrying a light rain jacket.
When a trip like this runs about $2200, paying for it upfront can sting. Spreading those costs out over a longer window makes the math way easier to handle while you are booking your lodges and excursions.
That 18-month window is a hard deadline, not free money, and any leftover balance at the end will get hit with regular interest charges.