Hi. In order to answer this question, you must first understand the meaning of the word, synecdoche.
Definition: A synecdoche is a figure of speech in which a part is made to represent the whole or vice-versa.
That being said, you can see that in the sentence you provided, it says, "...people used plastic to pay for everything". So, does this mean that they used a plastic bottle? Maybe a plastic toy? Obviously, the answer is no.
The word, "plastic" is often used to describe credit cards, therefore, the synecdoche in this question is, "...people use plastic to pay"
Hope this helps.
Take care,
Diana
