If your Avira account differs from your address, you have to change the delivery address in the final step when placing your order.
Otherwise, you will have 2 different Avira accounts.
Your existing account, and the new account with the purchased license that uses the same email address as your PayPal account.
Change the delivery address
Proceed with your order as usual.
In the final step, change the "Delivery Address" to your existing Avira account email address.