Automatic billing in prosites and whmcs instead of invoice billing

I'm working the WHMCS integration with prosites. Client gets a free website and then has the choice to upgrade to premium accounts.

Billing is being handled through whmcs billing, which runs on a cron job through my reseller account every 24 hours. so when I sell a domain or hosting or a sub-domain or a prosite upgrade the client pays via an invoice, which could go out to that client within 24 hours depending on when they signup.

Is there a way to integrate an automatic payment feature whereby the client is charged at the time of purchase/upgrade rather than through the invoice method?

Is so, how is that integrated? Is it through WHMCS using a payment processor or through ProSites using a payment Processor?