How to make the subscription a criteria for registration

[INCIID]

We are a nonprofit just switching our system of forums to VBulletin. Often when consumers register - they also donate. Is there a way to make donation/membership options part of the registration process. In other words, they have to select a subscription criteria in order to complete registration -- one of those would be free -- but others would be a designated donation amount. I see how to set up different subscriptions and we use Authorize.net to process --- but how can we connect the two - so the donation can be processed at the same time as registration?

Thanks

INCIID