For Bitdefender users with a subscription
From the Bitdefender console I was able to add another 5 devices under the MY SUBSCRIPTION page, I increased from 5 to 10 devices total for $9.99.
No code was needed for this, it automatically incremented the devices to 10 for the full remaining term of my subscription, 449 days. This includes the extra year I just purchased for $19.99 under another deal posted:
The end result was 5 devices/1year for $19.99 + 9.99 for 5 more devices = 10 devices for $29.98. Plus the reaming 89 days from my last subscription. YMMV