A few new ones (at least I hope they've not yet been mentioned - I may have missed some when looking over the thread) I've come across recently. As usual with these things, may not be available and/or couponable everywhere.
The first six Skulduggery Pleasant books by Derek Landy (HarperCollins) are available as two bundles (not in the US). Although the earlier books are mostly aimed at the 9+ crowd, with the later books getting an 11+ rating and I'd classify them as more YA than middle-grade, I'd say it's one of those series that crosses over well to any age range, provided you like a mixture of action, adventure, horror, fantasy and humour in a contemporary setting.
Skulduggery Pleasant: Books 1-3
ISBN: 9780007520749
Skulduggery Pleasant: Books 4-6
ISBN: 9780007520756
This is another MG/YA fantasy series omnibus:
Brandon Mull's Beyonders Trilogy - A World Without Heroes; Seeds of Rebellion; Chasing the Prophecy
ISBN: 9781442482388
The Mary Russell series by Laurie R. King (all 12 books out so far) has also been recently released as two bundles. I can't locate the first one on Kobo (it's
here on Amazon), but this is the second one, for books 5-12:
The Mary Russell Series 8-Book Bundle - O Jerusalem, Justice Hall, The Game, Locked Rooms, The Language of Bees, The God of the Hive, Pirate King, Garment of Shadows
ISBN: 9780804180481