How to create dollar volume discounts?

I need to create dollar volume discounts on top of customer discounts. Example: a reseller of our products logs into our site and can purchase a X% off of MSRP. If the reseller buys over $500 then they get an additional 5% (X+5%) discount...If they buy over $1500 they get an additional 10% off..(X%+10%).

However,

Some of our products have set pricing with no further discounts (NOt 40% off MSRP), so would need to not allow the further discounts on the products.

I did set it up where the reseller does get a customer discount, but if I put in 40% discount, it applies to the entire order, the problem is some products are not provided at 40% off.

So, I am not sure how to set it to calculate an extra discount based on dollar volume, but not apply to the dollar volume to all products...

Be willing to pay for someone to do this if they know how.

You might look at the Group Discount per Item add on where you can set the specific discount for a Group for a Product ...
http://www.zen-cart.com/index.php?main_page=product_contrib_info&products_id=256

I think this works up to 4 Discount Groups, but it might lead you to what you are trying to do and how to expand on it if needed ...