Classic cost-benefit analysis is mentally difficult, so humans frequently make decisions based on comparisons to what else is available. When a strategically designed additional option changes an individual’s original choice, that is the decoy effect. Formally, the decoy effect is called asymmetrically dominated choice.
To encourage someone to pick a specific choice, create a third option to improve the desirability of your suggested option.
If you want the client to buy the higher priced of two items, create a new item in the middle with a similar price but much less value. For example, create a “medium” popcorn for $1 less than the large but close to the size of the small popcorn.
