it's unclear as to whether you paid the $9.000 out of pocket or just couldn't collect on the load.

if you paid it out of pocket, you can deduct what you paid for it. if you incurred no out-of-pocket expense, you just couldn't collect on delivery there is nothing to deduct because you haven't included it in income.  

After the customer refused the load , the shipper wouldn't take it back, so the shipper charged me for load, which now became my load and I ended up disposing it.

Does this particular loss go under inventory / cost of goods, or business expenses?

It should go under Inventory.