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The cost is minor, but that's not the point. We dislike being taken advantage of.
A "not sure if serious" meme would be appropriate here.
Leaving anything (let alone something made out of plastic) on top of a hob is negligent.
As a result of your negligence, someone else's property was damaged.
That person has charged you
only the direct replacement cost. Replacing a set when one of the set is damaged is a direct replacement when the single damaged item can't be replaced individually.
They haven't charged you for the time and inconvenience they incurred as a result of your negligence, nor have they charged you for any other expenses they incurred as a result of your negligence. Delivery isn't usually free on very cheap items.
They haven't charged you for any of the usual vastly inflated "admin fee" type of thing.
They haven't charged you a fee for disposing of the damaged set.
They've been generous to you. Quite the opposite of taking advantage of you. Your negligence could have destroyed their cottage.
And alongside all of that, the sum of money involved is less than £15. Much less, really, since you're not disputing the largest of the 3 chopping boards and that would be more than 1/3rd of the cost of a set of 3 cheap plastic chopping boards with the other two being smaller. So you're this bothered about maybe as much as £9, tops. That is relevant. Cost is always relevant. A dispute over money only matters if the amount of money is at least slightly significant.
If you really want the set you damaged, then you should be willing to cover the cost of you getting it. Either travel there to collect it or pay for the cost to have it delivered to you. If the other party is being nice about it, they could waive the cost of their time. If they're being like you, they could add that cost on too. Either way, the fair cost to you would exceed the value of the damaged set. But it's the principle of the thing, right?
Have you considered challenging them to a duel over this serious point of honour?