ONLINE Double-Sided Fountain Pen Ink Cartridges, Blue, 20-Pack

ONLINE Double-Sided Fountain Pen Ink Cartridges, Blue, 20-Pack is down to £5.99 at Amazon. It has typically been selling for around £12.73 over recent months, so this works out at roughly 53% below its usual price.
Its currently ranked 2,172 in Stationery & Office Supplies on Amazon and rated 4.4 out of 5 from 5,328 customer ratings, so plenty of buyers have already tried it.
Key features:
Universal double-sided cartridges for common fountain pens, including Lamy
Four packs, 20 cartridges in total, in royal blue
Usable on both sides: one end fits Lamy pens, the other fits other common fountain pens
Extra-large cartridges, so they need changing less often
Made by ONLINE, a German pen company founded in 1991
That works out at about 30p per cartridge, which is handy for school or anyone who writes a lot.