zyBooks are steeply discounted for students, so most financial aid students initially subscribe directly using a credit card at zybooks.com, then "reimburse themselves" when their aid is disbursed several weeks into the term.
In rare hardship cases, we may defer payment for a few weeks. Email support@zybooks.com to request a deferment.
You might check whether your bookstore sells zyBooks prepaid access keys, in which case you might use your school financial aid account to purchase a key from the bookstore (Info about prepaid access keys / Info for bookstore representatives). Note that most financial aid students pay directly at zybooks.com, which may have a lower price, enable future discounts, and save time too, and thus most schools using zyBooks do not set up such prepaid access keys.
By the way, the first chapter is free, so you can immediately start reading and doing activities even before paying or requesting deferment. When in the chapter but unsubscribed, your activity will be recorded, but not visible to your instructor. After you subscribe, you will appear on that zyBook's roster, and activities you've already completed will become visible to your instructor. So you can get started learning right away.