Cannot Complete Purchase Online w/ Account Balance in Education Store

I am trying to use a combination of my account balance and a credit card to purchase an Apple Pencil online in the Education Store. When I go to check out, I apply my account balance and then input my credit card information for the remaining balance. When I click "Continue to Review," nothing happens (the button just turns gray with a spinning icon, and then returns to normal); there is no error message, and there's no indication that there is anything wrong with my payment information.

1. I have checked, and it says that account balances can be used to buy accessories in the online store.
2. I have tried to do this on two separate devices and with different browsers.
3. I tried to use a different credit card.
4. I even tried making the purchase in the regular Apple online store without the education discount.

Every time I get the same result. Any ideas? I've chatted online with Apple Support on three separate occasions over two days at this point, and they have not been able to offer any help. Unfortunately, I'm pretty far away from the nearest Apple store and can't just go in person.