I've seen a few of these posts now, but its getting a bit repetitive. The Argos ordering system is completely on the blink and any items that are either:
Buy one, get one free
Buy Two, save £xx
Buy Two for £xx
Are all glitched and quickly clicking the buy or reserve button adds then at prices much below what they should be.
Set of three seagrass boxes 875/2817 - BOGOF at £15 but if you add a few of these they go in your basket for free.
PC Games - £19.99 but buy two for £30 - If you add a few of these they go in your basket for £10 a game - effectively two for £20
Works on pretty much everything. Obviously there are no guarantees that any orders will be processed, and I seriously doubt any stores will honour the wrong prices, but there you go.