8.5 The Parties agree that in case the Purchaser challenges the Seller's invoice in relation to the Services or the Products, the Parties are bound to a two-month best efforts duty to settle amicably any dispute of thereof in good faith and that the accrual of legal interests for late payment should in any case not start prior to a period of two (2) months following the Purchaser's notification to the Seller of the challenge of its invoice...
13.3.4 on request, each Party shall certify in writing to the other Party that it has complied with the requirements of this clause.