Zwift gift card

Hi,
yesterday (10/8/2019) I subscrived a zwift month.
Today (10/9/2019) I redeemed a gift card I had with the roller.
I did it thinking that the subscription period was added to the one paid the day before.

What I thought does not appear on the order page.
How does it work?
I’m afraid I paid twice the same month.

Could you support me?
Thanks in advice.
Luca

Your best bet is to contact Zwift Support: https://support.zwift.com/contact