Here's why:
* Appositive: An appositive is a noun or pronoun that renames or explains another noun or pronoun. It's usually set off by commas.
* In this sentence: "People from all walks of life" renames and clarifies the noun "customers." It tells us the type of customers who buy merchandise at the discount store.
Therefore, the phrase "people from all walks of life" is the appositive phrase in this sentence.