upsell

As an adjective additional

is supplemental or added to something.

As a noun additional

is something added.

As a verb upsell is

to persuade a customer to buy more than he or she had intended.

As verbs the difference between upsell and unsell

is that upsell is to persuade a customer to buy more than he or she had intended while unsell is to cause to appear less attractive to consumers or adherents.

As verbs the difference between upswell and upsell

is that upswell is to swell upward while upsell is to persuade a customer to buy more than he or she had intended.

As a noun upswell

is a rising swell.

As verbs the difference between upsell and upwell

is that upsell is to persuade a customer to buy more than he or she had intended while upwell is to rise from a lower source; to well up.