Yes, your invoices for services rendered to the estate should be treated like any other invoices for services, materials, good, etc. provided to the estate, assuming that (as you imply) there was an agreement in place between you and the executor to pay for your services (i.e. if you initially "volunteered" your efforts without an agreement for compensation, you cannot after-the-fact force the estate to pay). If the estate does not pay, you could sue the estate for the money, such as in small claims court on a "pro se" (as your own attorney) basis.