How can I tell is a payment solution like Sezzle is helping sales or just getting sales that would have come in anyway?

So, I've got an ecomm store and we just rolled out "Sezzle".

This is a payment processor that pays me now for customer orders, and then lets the customers make payments later to cover the cost of the goods.

I wind up paying 6% of the gross order value instead of the 3% or so that I would normally pay to the credit card companies.

Sezzle wound up being 14% of sales in July.

Any idea how I can determine if these orders would have come in anyway?