I think you have a GM S-Plan. S=Supplier. I am a member of the S plan as well.
I do not know the % of discount, but it is definatly worth using. I saved about
$5000 + on a GMC 1500HD. Sticker price on mine was $38,000.00 and it got it for
$ 28,000.00 (using S Plan, $ 3000.00 cash back & $ 1,000 owner loyalty cash)
Ask to see the dealer's invoice, I think it is written somewhere that they must
show it to you. On the invoice look for the "S" and that should be your cost +
any dealer added options.