Final answer:
Melody earns a commission, which is a type of income dependent on the amount of sales she makes.
Step-by-step explanation:
The type of income that Melody is earning at her job in a department store, where her pay is based on the amount she sells, is referred to as commission. This compensation structure is common in sales positions where employees are incentivized to sell more products or services. Under a commission-based income system, the employee usually receives a certain percentage of the sales they generate, so their earnings tend to fluctuate with their performance.