This Misprice deal seems to be store specific. I have checked again today 24/01 & plenty in stock.
4 Brita Maxtra Cartridges £5.00 at Sainsburys (Instore).
As per price tag: £10.00 (Half Price, was £20.00)
At checkout I got another £5.00 off. That means final price for 4 Brita Maxtra Cartridges £5.00.
It seems it is misprice as the half price applied twice (Was £20, then £10 at till anothre £5 off => final price £5)
The Store reciept shows -
*BRITA MAXTRA 4PK WA £10.00
Brita max4pk cart 50 -£5.00
Plenty (at least 30) available in Apsley mills Sanisburys. .