Gross and Net Copper Commodity pricing

This is my first post.
I could do with some insight into what I am getting wrong on something,
I am learning about trading commodities and specifically trading copper, something I am new to.
I have a (potential) client who said the following :

I need copper cathodes
I need a supply of cathodes for my buyer in china CIF
volume 10000 tonnes + per month
payment DLC seller 2% PB
price -20 % gross -17 net

So I found a supplier of Copper Cathode at 3000 US / MT and I get 8 % commission (which I inevitably have to share)

Current price of LME Copper Cathode is 5800 USD / Ton

So a 20% discount is 1160 USD (on 5800 USD) so the price would be 4640 USD

So offering the copper at 3000 USD per tonne is better than 4640 I thought he would be pleased.

Why is it discounted? as the cathodes are not LME certified.

Instead I received an email saying the following:

You said you had a copper cathode offer that the price was equal to or better than -20 LME
and the offer you sent over is - 8 LME
so im confused ?

Why is the client confused? What am I getting wrong please :)
And what is the significance of the difference between the gross and the nett I had assumed that is where the commission came from of the buyers side.

PS don't shoot me into orbit too much I am certain I am getting something very fundamental wrong and I have read just about everything I can find on the internet but I still cannot fathom out my mistake.

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