Probably a pricing error, so orders may be cancelled.
It can be fun to give it a go, and it's very unlikely the shop will charge you full price, but don't be too disappointed if it doesn't work out! Here's what Citizen's Advice has to say on it.
Half price kids fancy dress, perfect for book week dressing up at school.
Glitch maybe on Argos as discount only showed up when paying. Bargain
Delivery and returns at Argos
Click and Collect is Free.
Standard delivery is £3.95, and delivery for large items starts at £6.95.
Returns - You have 30 days from the date of purchase to return your items.
Refunds will be processed by the original payment method. If you paid on card, it can take up to 7 days to appear back in your account, with PayPal refunds taking up to 30 days. You can return your items to an Argos store, a Sainsbury's Collection Point or by post (check your delivery note)