Boxing Day is not and has never been a shopping holiday in the USA; however, 26 December often starts many U.S. retailers' after-Christmas sales and the earliest starting day after Christmas of many people, ex. Americans, returning unwanted gifts for exchanges or refunds.

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What is the meaning of "ex." in the last sentence? How do you read "ex."?

Example😑

Yes, ‘example’ (meaning, ‘for example’).

The abbreviation "ex." in the last sentence stands for "example." It is often used to introduce an example to support or illustrate a statement. When reading "ex.", you can simply say "example." So in this context, "ex." means "for example" and is used to indicate that Americans are one example of many people who may return unwanted gifts after Christmas.