Do you offer a one-time payment option?
While there isn't a way to pay for one or just a couple months at a time, or a per survey fee, a monthly plan might work for you.
Monthly plans let you pay on a month-to-month basis. Be sure to understand auto-renew before your renewal comes up so you're not charged for your next month unless you need more time. You can cancel a monthly subscription from your Billing Details. If you decide to cancel, your surveys and data will be stored in your account—you'll still have access to all the features of the free plan, and you can re-upgrade at any time if you need to use paid features in the future.
One thing to know about monthly plans is the 1,000 responses limit. While annual plans let you collect unlimited responses at no extra charge, if you collect more than 1,000 responses in a given month on a monthly plan, you'll need to pay overages.