I further certify that if any property so purchased tax free is used or consumed by the firm as to make it subject to a sales or use tax we will pay the tax due direct to the proper taxing authority when state law so provides or inform the seller for added tax billing. This certificate shall be part of each order which we may hereafter give to you, unless otherwise specified, and shall be valid until cancelled by us in writing or revoked by the city or state. General description of products to be purchased from the seller: