The Boxcar Children Mysteries Box Set: Books One Through Twelve on sale today for
$3.99*. (Appears to be US only.)
*For some bizarre reason, it's being listed as $3.03 on Google.
This is an Open Road Media book and couponable at kobo.
I was able to use the 75% code on the front page of the kobo discount thread successfully with this. Though, this series is well worth the $4 without it.
Kobo US
Amazon US
Google US