They have several flexible ways of paying. Here is a link to some info: Thomas Edison State College | Selecting the Right Tuition Plan Installment plans imply some form of credit. You could always get a subsidized student loan and pay in any installment you would like. If it's subsidized, you will not incur interest until you finish your degree. You could pay it off as though it were an installment plan if you pay each month while attending school, as much as you can afford. If you pay as much as you can while attending school, you may can have it paid off by the time you graduate. I had my master's degree paid off soon after I graduated and it's a good feeling.
From the linked site: "Are you a part-time student? For students who plan on earning their degree on more of a part-time basis and earning less than 24 credits in a year (eight courses or less), our Enrolled Options Plan is the best value. This option enables students to enroll and pay for courses at their own pace." That sounds like installments to me.
If you would have looked more closely at the Enrolled Options plan (as Kizmet also said) you would have found the closest thing to an installment plan. Also, why not just call the school and ask?