First read the fine print. If you have to pay it, pay it and switch company.
If you don't have to pay it and there is no proof that you abused the component beyond normal usage, you don't have to sue them, just return the invoice with legal (not so layman) text like "I hereby reject paying invoice number xxxx dated xxx because the black box was used under normal conditions and it stopped working".
In this case you wait for them and answer every other letter with the same text until the decide to either sue you, or drop the whole thing.
If you choose this path, remember to save all invoice, copies of your rejections, all written/email/phone calls, picutres of the broken item, serial nubmers, contract etc.
If they sue you and they loose (can't prove the item was destroied by you), they have to pay you up to one hour of legal advice cost and drop the invoice, if you loose, you do the same (100 pounds) plus the invoice amount according to Swedish law, don't know about your country.
Before you follow any advice here, consult your local consumer protection agency, they usually comes up with smart options, they know a bad company with history and give you the right advice.