Volume discounts sell more of one thing. Bundles sell a different combination of things.
Volume discounts increase quantity of one product; bundles change what combination the customer buys. These suit different catalogues.
Volume works where a second unit has genuine value — consumables, refills, gifts, anything routinely bought in multiples.
Bundles work where products complete each other, and they let you attach slower stock to something that already sells.
Both cannibalise if applied to what customers were buying anyway, so measure against a control rather than counting mechanic revenue.
A volume discount reduces the price when a customer buys more units of the same product — buy three, save fifteen percent.
The full guide has the formats, the deadlines and the worked examples.