I called Sandisk's helpline since I have purchased quite a few cards. All the cards can be on one invoice. You need to copy the invoice to accompany the bar codes cut from the boxes for a maximum of 3 bar codes per redemption.

Incidentally we tested all 80 500GB Nexto Extremes today by formatting them, copying a file from a Nano and formatting them again. We found one unit which made a clicking sound but worked, and they are are replacing it. Not bad.

Jeff