If you're eligible for instant credit, you can apply the credit to your online order. If you mail in your device for appraisal and are not receiving instant credit, you'll have to wait for an account credit.
You can use your instant credit to:
Purchase accessories
Purchase a new phone under a device payment agreement (in limited states) or at the two-year price
Note: If your fair market value instant-credit trade-in device is in different condition than expected when we receive it, a chargeback for items in worse condition or credit for items in better condition will be applied to your bill.
You can use your (non-instant) trade-in account credit to:
Apply toward your bill if you have a standard monthly account
Purchase accessories
Purchase a new phone under a device payment agreement (in limited states)