Seller wants an extra £250 to process my order

[adralerh85]

I bought a pergola for £389 a few days ago.

The next day i received a message saying that instead of it turning up on the 17/6/21 as advertised, that it would be delivered in 15-20 days. I was willing to accept this as they said it is being made to order.

Today the seller is asking for an extra £250 as they say that the costs of the wood have sky rocketed recently and that they would now be at a loss if they sent me the pergola.

Are they allowed to ask me for more money after i have already paid?  I thought that it was a contract or am i wrong? 

i compared it to going to the supermarket and picking up a loaf of bread at £1 only to get to the till and be told that the price of flour has gone up since i picked the bread up and now if i want it i'll need to pay £2.

Im going to cancel anyway but im curious to know who is right and wrong.

Thank you to whoever can point me in the right direction.