Yes, you have to pay, and no, your letter indicating that you considered the account closed has no effect whatsoever. He can effectively invoice up you up to three years after the work is done. After three years, if you refused or failed to pay, he could not longer bring a legal action to enforce the debt or contract, since the statute of limitations would have expired--therefore, there'd be no point in invoicing after three years, since at that time, you could just ignore the invoice.However, until the statutory period (3 years) is up, they can invoice you and you have to pay, or else risk being sued. Your statement that you consider the account closed is irrelevant because neither party can unilaterally affect the rights of the other.It is certainly unusual to wait so long to invoice, but as stated above, it is not illegal, and the invoice is still enforceable.