I am trying to print a label for a book that weighs 12.5 ounces. I am getting a message saying the book is over 13 ounces and cannot be mailed from the post office. Am I correct that as long as the book does not weigh more than 13 ounces I can mail it from home with stamps?

The weight of the complete package you are mailing must be less than 13oz.  PBS adds a bit for the weight of the packaging, which could take it over.  Always make sure you weigh the package you are mailing, not just the book.

When it comes to postal weight, you MUST ROUND UP.  You claim the book weighs 12.5 ounces, so you must use the 13 ounce rate.  Even if it weighed 12.1 ounces, you would still use the 13 ounce rate.

Also, you need to use the weight of the entire package, not just the contents.