It may take a little time depending on the payment method used for it to clear ?

Once cleared your email with license file will be sent.

In the meantime you can download and install the version you paid for and start using it on the trial (30 days unrestricted use) version. So there is plenty of time to receive it, before you need to enter this license.

If you haven’t already downloaded the version you paid for, here are some direct links (I don’t know what version you paid for).
Pro AV: - http://files.avast.com/iavs5x/avast_pro_antivirus_setup.exe
AIS: - http://files.avast.com/iavs5x/avast_internet_security_setup.exe
Premier: - http://files.avast.com/iavs5x/avast_premier_antivirus_setup.exe

If you have a current anti virus (?), download the avast version, uninstall the existing AV, reboot and install avast.