Yes! If you already subscribe directly through SHOWTIME.com you will be able to apply your SHOWTIME gift to your existing account as a credit. Once applied, the cost of your subscription will be deducted from the available credit. Any remaining credit will be applied to future subscription periods until the credit has been used up.
Gifts may be redeemed only towards an individual subscription to the SHOWTIME standalone streaming service in the United States. SHOWTIME gifts cannot be applied to subscriptions billed through other billing partners (Amazon, Android, Apple or Roku), add-on subscriptions through Hulu, Amazon Video, PlayStation Vue, Sling TV, or YouTube TV, or subscriptions through cable, satellite or telco operators.