I think the risk to reward ratio is too great for buyers to take a chance.
It would have to be for the most well established customers who (in your case) regularly send in large loads, where a high percentage of the items test good for more than scrap value.
I have come to accept the fact that if I sell to various buyers where I need to ship items, there will be a delay in getting paid. That is just the nature of the beast. If I need to get paid right away, my sale is going to be made locally and is going to be for scrap value only.
Bottom line is that if I want more than scrap value and more than the local yards are paying, then I need to wait. Would I be tempted to try a buyer that offers pre-pay, yes. But I think any buyer that offers that would probably have to lower prices a little to make up for the risk they are taking.