Ahh sorry Hooper you were posting "tangible" the same time I was posting my suggestion.
Rolo's advice above sounds right on the money.
Also if you haven't done it yet, you might want to include a page with the invoice and product that outlines your return procedure, mailing address etc, or even a "product registration" card. If they return the card then they're registering the product and are evidently happy with it, yes? Might be more ammo in the event of a future chargeback.