1. From the Pick & Pay screen 

You can see the amount of Store Credit a customer has in the payment section on the right.

2. To apply the Store Credit

Simply enter the amount you wish to apply in the text field to the right of the Store Credit line item. The system will automatically deduct the amount of Store Credit applied, from the Amount Due. Note in the example below, $10 of the $16 in Store Credit available have been applied to this payment. The subtotal is $60.37, less the $10 in Applied Store Credit, leaves an Amount Due of $50.37. The Balance line item is the Store Credit Balance, we applied $10 of the $16 available, leaving a Store Credit Balance of $6.00.