You can pay monthly, but it comes at a higher yearly price. Since we're users and not Adobe staff, and because there are many different choices, you probably should contact Adobe:
It's my understanding that the yearly plans are also billed in monthly installments.
You can prepay for a year, but I think you have to that through a third
party. You can buy one year subscriptions at Staple, Best Buy and other
retails. Keep an eye out for sales at those store.
Staples emails coupons periodically good for up to 25% off.