If you sign up for Amazon Mom, you get Prime free shipping benefits for a few months, and the time extends each time you buy a certain amount of qualifying purchases. (Most of these are kid and baby items, but sometimes there are toiletries, undergarments, and other random things).
The key here is that there is nothing that says you need to be a mom to sign up. You do have to select from a drop down menu that includes choices like "Parent" and "grandparent", but one option is "Family Member". I am, in fact, a member of a family, so I selected that.
This can save you shipping costs if you are going to make some purchases over the course of a couple months, or for even longer if you are able to keep the benefits going (but only if you don't purchase anything you otherwise wouldn't have in order to get it!).