What is the term for a special service a firm provides to secure a sale?

Any place I ever worked always called it your binder

Look up sales binder

Depending on which type of sales, realtor, mortgage broker, insurance agent, pharmaceutical sales rep, binder can be used different ways

Perks.

The perks of this vacuum are free bags and maintenance for one year.

Perks:: noun INFORMAL

money, goods, or other benefit to which one is entitled as an employee or as a shareholder of a company. "many agencies are helping to keep personnel at their jobs by providing perks" an advantage or benefit following from a job or situation. [Dictionary.com]

The dictionary definition is not too convincing, but I've seen this used frequently in sales discussion.

Incentive(s)

Oxford dictionary defines it as

something that encourages you to do something

An example of which is

There is an added incentive for you to buy from our catalogue—a free gift with every purchase.