Terms (from the Spotify website):
Spotify gift cards cannot be used as a valid payment method for the Spotify Family Plan. If you currently subscribe to Spotify through a third party (such as a telephone company) you will not be able to take advantage of this offer. If you decide that you do not want to remain a paying user of the Spotify Premium Service, you must cancel your subscription to the Premium Service by logging into your Spotify account and following the prompts on the Account page, or by clicking hereand following the instructions. If the primary account holder cancels the Premium subscription, all linked sub-accounts will revert to the Spotify Free Service.
No other discounts can be applied. There are no refunds or credits for partial monthly subscriptions. If Spotify increases the monthly fee in the future, we will provide you notice. Price changes will take effect at the start of the next subscription period following the date of the price change. By continuing to use the Spotify Service after the price change takes effect, you accept the new price. The Spotify Service may not be available on certain personal devices.