If your class is the first in a series of classes (such as General Chemistry I), it may be eligible for a multi-course discount. When you reach the payment screen for your course, click on the All Semesters option. This will purchase the current term's course, and will also apply credit to your account to use toward a future term.
The discount only applies if you buy multiple terms up front. In other words, if you buy a single term now, you cannot receive the discount when you purchase the next course in the sequence later.
If you use a prepaid access card, it will automatically give you credit for one or multiple courses depending on the type of card you purchased.