Can't Purchase Webinar Add-On

Is anyone having this problem?  On multiple computers and multiple browsers, every time I try to purchase the Webinar upgrade, I get this:

Cannot invoke "com.zuora.api.axis2.ZuoraServiceStub$PaymentMethod.getMandateID()" because "dfPaymentMethod" is null (-1)

It's been a few days of this and I really need to get Webinar to set up a meeting for a client.  Ideas?