You can imagine how much it would improve postal fortunes to rescind that exception eh. But, we digress.
So, under the law Wal-Mart continues to OWN THE RECEIPT (or "letter") and the ownership NEVER conveys to the recipient.
I know that's too esoteric for you, but you will have to handle the receipt in a way required by Wal-Mart until you exit their premises and drive away out of their parking lot. Then, if they didn't snatch it back from you, you could dispose of it as you wish, but you still don't own it.
This business about who OWNS a stack of letters was settled ages ago through a series of statutes and court decisions, and sometimes there are disputes ~ particularly when a famous sender dies! But such things as receipts with personal information (e.g. part of your credit card number, time, date of purchase, location, price) makes them "personal information", which, if mailed, subjects them to First Class Mail rates ~ just like any other personal document.
Always make sure that you have a "release" from the owner in hand before attempting to sell your souvenir collection of "bills sent to Elvis" (e.g.).
Let's not. Let's You provide a proper response on natural law.