Is this legal?

WeAreGhosts

https://www.facebook.com/colourwithclaire/posts/500096628151029

You could win a competition but have no idea how much postage will cost you? Never heard anything like that.

Apparently the comp will now close early due to the organiser getting overwhelmed with numbers 
https://www.facebook.com/colourwithclaire/posts/500671974760161

Ms_Dee

Most larger companies state that they can move the competition closing date if needed/wanted in their terms and conditions.

Lots of competitions also state you must be willing to pay postage, I think this person hasn't stated postage costs, as they might be using a courier where the price depends on the postcode to postcode.
MSE do not encourage people to spend any money on competitions or postage though, as it is often not really a prize.

I've seen worse run competitions and plenty that don't stick to the rules, especially Facebook terms and conditions and the conventional gambling laws, this doesn't stop larger companies either, sometimes they are worse.

I think it would have been much better if the host announced that the winners may be selected a day later, rather than closing the whole competition. There was also nothing stopping them writing people's names on a list once everyone has entered, as people could still remove their comments during and Facebook often removes comments for spam too, so commenting on every entry with a number doesn't always work, plus it would also be easier for them to see any duplicates that way too, as it is one entry per person.

I'm not sure if everything is above board, but it certainly could be handled a little more professionally. They say postage will need to be covered but they don't say if packaging will need to be covered as well. You could probably get some of these book secondhand cheaper than the postage they may be charging, as most of these books are selling for around £5-£10 on Amazon and they have also stated: 

A huge lot of 40 books has been donated to my channel for giveaway! 99% are new and uncoloured; there are a couple of books where a small element has been coloured (eg. an emblem on the 'belongs to' page).

Which I'd classed as used, so probably not a competition I'd particularly be interested in entering.

Some useful links:
https://www.asa.org.uk/advice-online/promotional-marketing-competitions.html
https://www.gamblingcommission.gov.uk/public-and-players/guide/page/free-draws-and-prize-competitions
https://www.facebook.com/business/news/page-promotions-terms

mjm3346

Closing dates should generally not be moved

"Closing dates must not be changed unless unavoidable circumstances beyond the control of the promoter make it necessary and either not to change the date would be unfair to those who sought to participate within the original terms, or those who sought to participate within the original terms will not be disadvantaged by the change"