After your free trial, you will not be charged unless you decide to purchase a subscription for the product.
When your trial expires and you log into the Universal product, you will be able to purchase the product with a credit card. If the Universal product did not meet your needs... no action is necessary - thanks for giving it a go.
If it does work for your students and you decide to purchase it, you will be charged immediately for your first month of your subscription (often called pre-billing). You will be billed every month until you choose to cancel the subscription.