Well it can't be a ponzi scheme, you don't keep money in it.
It can be a ponzi scheme if they are using money from new orders to fund old orders, which is exactly what has been going on with them for several months now.
They say zipzap doesn't have a backend and they had to find a developer to do the api call.
That is laughable nonsense. For starters, I have a ZipZap merchant account, and they do have a back end (the back end is hosted on CashPayment.com). Second, they may choose to do refunds via API call, but if they do, one would think that given the 409 pages of complaints in their support thread, they would have done these refunds before and have already had the code written for making them.
Yet another lie they have been caught in trying to buy time to get more orders in to pay for old orders.