While we used to allow invoicing for all subscription plans, we found that there was often a disconnect between the subscription being taken out and payment being received. This often resulted in payments we couldn't identify and subscriptions remaining in a pending status for months on end.
Where we previously used PayPal as a payment processor, which was problematic for some business card holders, we are now using a new payment processor that should elminate the problems. As such, we are now only accepting card payments for Single User and Lite subscriptions.