A bunch of Annabel Chase box sets are $0.99/£0.99 in the US/UK right now. Some have been at that price for a while, and some are more recent price drops.
Federal Bureau of Magic: Box Set 1-7 - a box set of books 1-3 was on sale a while back, but $0.99 is still a good price, even if you only need/want the next four
Kindle US:
https://www.amazon.com/dp/B0F1XDNVKV
Kindle UK:
https://www.amazon.co.uk/dp/B0F1XDNVKV
Spellbound Box Set, Books 1-10 - most or all of these were free on at least one occasion a while back, so check before you buy
Kindle US:
https://www.amazon.com/dp/B0CPLH1PBQ
Kindle UK:
https://www.amazon.co.uk/dp/B0CPLH1PBQ
Spellbound Ever After Box Set: Books 1-9 - and most or all of these were also free on at least one occasion a while back, so check before you buy
Kindle US:
https://www.amazon.com/dp/B0DWJ6BDZP
Kindle UK:
https://www.amazon.co.uk/dp/B0DWJ6BDZP
Spellslingers Academy of Magic Complete Series
Kindle US:
https://www.amazon.com/dp/B0CWFJLHHP
Kindle UK:
https://www.amazon.co.uk/dp/B0CWFJLHHP
Midnight Empire Complete Series
Kindle US:
https://www.amazon.com/dp/B0DTGY6PP4
Kindle UK:
https://www.amazon.co.uk/dp/B0DTGY6PP4
Starry Hollow Witches Box Set, Books 1-10
Kindle US:
https://www.amazon.com/dp/B0D6X226YY
Kindle UK:
https://www.amazon.co.uk/dp/B0D6X226YY