This support request is being escalated to our root admins who have more visibility to dig into this issue some more.
In the meantime, could you please provide the PayPal transaction ID, which may help them find the payment quicker?
An email alias just forwards an email to another email address. You cannot send mail directly from it. If you need to send emails from it, then you would need to create the email as a separate email account instead of an alias.