it's a mail-in rebate and on the form it says;
"...The computer and printer must appear on the same invoice or sales receipt...
...Customer rebate will be equal to the amount invoiced up to a maximum of $100..."
so if you buy a printer that's less than $100, the rebate amounts to the price of the printer and not the full $100
and, yes, the Apple store had at least one model that was less than $100