Three ways to shape what customers buy: add-ons sold alongside a service, bundles sold together at one price, and promotion codes at checkout.
Product add-ons
An add-on is bought alongside a service, with its own price, billing cycle and status. Suspending the service suspends its add-ons; terminating it terminates them. Restrict an add-on to the products it makes sense for, and invoice it on the parent's cycle or its own.
Bundles
Several products sold together at a combined price. Copy an existing bundle and adjust the copy rather than building a variant from scratch.
Promotions
Codes customers enter at checkout: a percentage, a fixed amount, a price override or free setup, with rules for who may use them, which products they cover and how long they last.
Restrict a promotion to products
A promotion with no product restriction applies to everything, which is rarely what you meant.
Full guide
The complete reference is Plans, services, and orders in the WBAMS documentation.