I have a similar need.

Not only do I sell hosting, but also connectivity. My service provider has now stipulated a 30 days cancellation noticed which excludes current month. In other words, if today is any day of March and I terminate with my supplier they will only terminate end of April minimum. However in my WHMCS my client can terminate immediately or end of billing period. This means that I have to pay an additional month with y supplier, but no legal binding with my client as we cannot force terminate dates based on specific products.