I have used them in ornamental gardens successfully with wood chips over them along with regular brown cardboard. I haven't used them in the veggie garden.

Thanks. I found the answer to my own question. Cardboard is good for blocking weeds, or so I hear from many posts around the Internet.

I looked at the USPS site and it says that the Priority boxes are made for cradle to cradle. That means that they are fully recyclable. Good news for the garden if anyone wants to get a good weed barrier for free. Don't go crazy though or USPS might charge us for them.

Those are tiny boxes. You could get much larger cardboard from a furniture or appliance store.

I agree with purple. The bigger the sheets of cardboard the better this technique works. Weeds have a way of finding the joins. And it doesn't work on bindweed in the long run. It'll find its way to a weak spot and reappear. I use cardboard with leaves on between raspberry rows. It only lasts one season and needs redoing every year. But it does help.