Payment certification and supplier undertaking require site acceptance, invoice payment confirmation, and mandatory refund notification to authorities. Two prescribed forms record supply and payment compliance: Form I certifies contractual supply, conformity with specifications, acceptance at project site, inclusion of sub contractor in the main contract, and that payment was made at invoice price by an authorised project official; Form II is an undertaking by supplier/recipient confirming application particulars, contractual supply/receipt and payment, with an obligation to notify the Regional Authority within thirty days of any refund or return due to non satisfactory performance and to refund proportionate amounts to the Authority.