The simplest and most common structure: buy this much, earn this rate. Complexity arrives with the definition of "this much" — which entities count, which products count, whether returns net off, and how volume is measured across ship-to locations that belong to one commercial customer.
That last question is where volume rebates most often fail: the customer is one relationship and many identifiers, and unless those identifiers are cross-referenced, the tier is evaluated on partial volume and the customer under-earns.