Earlier quoted context omitted.
It's worth noting that "Standard Mail" has a specific meaning here, and refers to things sent in bulk, like newsletters and flyers. It is emphatically not the standard service you would use to mail your aunt a letter. That service is called First Class Mail, and is not to be opened.
That's incorrect. "Standard mail" is the type of mail service used for anything that isn't one of the other named mail services, such as First Class or Priority Mail. Things sent in bulk, such as flyers, are an example of standard mail, but that does not preclude anyone from using standard mail to send a normal letter.
Perhaps you have a lot more aunts than I do, but everything I can find suggests standard mail had a large minimum (200 pieces?) and preprocessing requirements.
Here’s a page, from the post office itself:
“At one time, it was called Third-Class Mail. Today, it's known as Standard Mail. In 2017, it will be called (USPS) Marketing Mail. The U.S. Postal Service has proposed a name change for Standard Mail to better signal to customers that this mail is used primarily to market a product or service.”
https://www.uspsoig.gov/blog/names-and-prices-they-are-chang...