Jasmine, I am not sure how large an order you are talking about but in my custom order world, I would expect full payment up front. Just like any purchase in my shop.
Once you commit to taking custom orders you have to be able to produce, so if that means getting more supplies before you start, you have to cover your bases.
I would also give her an expected date of completion before letting the sale go through so you can come to an agreement on that ( and also give yourself the time to make the items)
If you don't already have a refund policy in place, I suggest you put one in so that you do not get left holding a custom order that might not resale .
Is there a reason that she might WANT to return the items? If it is a new design that you have not made before, why not charge her for only one, a prototype, and offer to send it to her before completing the entire order.
Good luck in this learning experience!