Seems like a great price for a 5 year license. According to AV Test's latest rating this product did extremely well.
Use the coupon code specified to get 4% back as account credit on G2A (pathetic I know), better than nothing.
To activate your key, go through this link.
If you have Windows defender setup correctly with AV/firewall/ransomware protect, and you don't really visit dodgy sites or DL high risk apps - is there any need for aftermarket AV in Win 10?