Retainer based consulting

After over a decade on my own and dealing with a very slow or non-paying client on average about once per year I have decided to switch to a retainer model. The gist is this, on an hourly engagement (most of my work) the client will pre-pay for hourly blocks of time - say 100 hours at a time and I will bill time against that block of hours. If it runs out, work stops until it is filled back up. If the work completes and funds are still in the account I cut a check back to the client.

I use Freshbooks for invoicing but this model does not fit, theirs seems to be based around the 'invoice after services' model. I am looking for a system that dove tails well with the retainer based model where the invoice is less the focus and it is more about the balance of the retainer account.

Any one know of a service that fits?