I just went to Target and the clerk told me it was $5 for the disk, $5 for the reservation, and $10 for the link cable = $20 total. Then I could use the reservation coupon for the cable bringing my total down to $10. As confusion is my normal state I said 'OK'. and proceeded to her register.
She rang the disk up for $5, realized there was no way to ring up the reservation card, and then rang up the cable. Then she took off the $10 and $5 coupons right there and said 'Thanks, have a nice day'. I asked her if she was sure i didn't have to pay anything and she confirmed that it was free. YMMV, but I think she made a mistake and I got a freebie.
Oh, yeah, she also gave me the GBA-GBA link cable and said that's what was part of the promotion. I didn't question it a second time as I got everything gratis.