You can choose between glossy or matte finishes at checkout before you submit your order.
You'll find the Prints Finish options right after you choose the delivery mode.
Please note that the matte finish is only available for shipped orders. Should you tap on the matte option, the delivery mode will automatically be switched to mail shipping.
However, if you retap on the glossy finish, the delivery mode will not be automatically returned to in-store pickup as glassy prints are also available for shipped orders.