Yes! If you already subscribe directly through SHOWTIME.com you will be able to apply the entire amount of 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 can only be applied to an individual subscription to the SHOWTIME standalone streaming service in the United States. SHOWTIME gifts cannot be applied to subscriptions started through other billing partners (Amazon, Android, Apple or Roku), add-on subscriptions through Hulu, Amazon Video or PlayStation Vue, or subscriptions through cable, satellite or telco operators.