Are you doing an "A" or a "B" shop? If it is "B", then it generally needs to be flat so as to fit into the Ready Post envelope. You can send them a pile of blank paper. Weigh it in advance. The envelope will add a little weight, depending upon which one you pick. Put the paper inside an envelope or even a plastic supermarket bag so it isn't obvious what it is if you are observed. Never say it is a book, since it triggers a special mailing rate. Same with magazine. Tell them that they are old photographs if asked, but rarely would you be asked anything other than is it a book or magazines or bound printed matter. Just say no. You can ship a book, or anything else.
If it is an "A" shop, then you have put it into a box in advance, and the box weighs a few ounces. Ship your parents a pound of rice or pasta or something else convenient. Or old magazines. Or something you really want to ship them, but keep it under two pounds.
Your parents of course need to be in a different town with a different zip code. The postage reimbursement will cover up thru Zone 4 if it is less than 2 pounds (and over 18 ounces). Be sure to weigh it in advance.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008