Heads up to anyone buying a time card:

I just found out you can't redeem a time card if you're on an Entry Subscription (standard, month to month $12.99). You have to upgrade to a Standard subscription, and in order to get the same rate of $12.99 per month you will need to get the 180 Day subscription.

After upgrading to the 180 subscription and entering my time card, I was able to switch back to an Entry Account, but I'm honestly not sure at this point how much I'll be charged by SE.

I definitely would have paid for those 180 days eventually anyway, but it's still an annoying headache that you're better off avoiding if there's some other video game item you want from Amazon.