If a student has signed up for a payment plan and would like to pay off the remainder in full, you can charge them in Stripe directly.
Create a New Payment in Stripe
First things first, you'll need to manually charge the student for the remainder of the payment plan. To do this, you'll need to create a new payment for the total in Stripe:
How to Create a New Payment in Stripe
Add a Credit in Stripe
After creating a new payment, you'll need to add credit in Stripe to ensure the remaining payments from the payment plan come out of the credit instead of charging the student. In the customer details in Stripe, click Add balance adjustment to add a credit.
If you have any questions about completing these actions on the Stripe side, you can reach out to them directly: Stripe Support