With this plugin you can have quantity discount on 4 levels.
These prices will overrule the default Salesprice 1 till 3 and can be set per debtor or creditor.

You can also set a special price for specific debtors or creditors per product, which overrule the default productprice.

These also a special field available in which you can add the suplliers product nummer.
Also possible for multiple suppliers.
This field can also be used for information text why this debtor/creditor has a special price.