Amazon Payment and User registration

[wolfgangs]

Hi there,
Virtuemart 3.0.18, Joomla 3.6.5
My virtuemart enables to buy a user group in my joomla:
When a user buys this user group by using paypal, he registers in my joomla during the purchase process. After paypal returns by ipn that the status of the product is completed, this user is assigned to the user group by the help of a custom plugin. No staff is necessary for this process.

The client for whom I made this virtuemart shop, would like to use amazon payments for his shop system too.
I believe, this does not work without the use of staff. A user, who is paying with amazon payments, does not register in the virtuemart shop, and so he can not be assigned the user group without the use of staff.
Am I wrong? Is there a settings in amazon payments plugin to force a user to register in joomla? Do you have any idea, how to solve this problem.
Thank you :-)
Wolfgang

[GJC Web Design]

why not force them to register during the checkout process?

Or does Amazon pay allow payment before the address stage is reached?