Returns and exchanges are up to the discretion of the merchant. Rather than processing a full refund, they may opt to issue a store credit.
Please keep in mind that your remaining Zip installments are still due even if you receive store credit from Fashion Nova. Your E-Gift Card or store credit code will be sent to the email on file for the order in question (this will come directly from Fashion Nova).
You place an order for $100 for various merchandise. You decide to return the whole order because the clothes didn't fit as expected. Fashion Nova will provide you with an E-Gift card once they have processed the return on their end.
But the Zip order will still need to be paid in full. If Zip were to refund you as well, you would be getting a store credit and a refund back to the card used for the order.