Would anyone be able to provide me more information regarding the payment plan or what it was like if you’ve personally gone through it? The website basically says nothing about it and I tried talking to an agent online about it and they gave me little to no information either basically saying I have a large sum due next month. Like HOW is that a payment plan then? 
I have enrolled in payment plan one semester and what i observed is that there will be three deadlines, one in each month and there will be a minimum amount that has to be paid before each deadline(which is usually 1/3 amount i guess). So, you are good if you pay the requested amount during each month and the entire amount by the last deadline. You can also check in this link about the due date and the enrollment fee:
If you still have further queries i would suggest you to call ISSC. They would give you more info. Take care !
I agree with what nerella said. One more suggestion would be to speak with the administration department who handles fee payment