II.18.2. E-invoicing  II.19. PRICE REVISION  II.19.1. Yearly price revision  II.19.2. Date of effect of the yearly price revision  II.19.3. Procedure for calculating the yearly price revision  The contractor (or leader in case of a joint tender) must complete the necessary formalities with the relevant  authorities to ensure that the supplies and/or services required for implementation of the FWC are exempt  from taxes and duties, including VAT.  The contractor shall have sole responsibility for compliance with the tax laws which apply to him. Failure to  comply shall make the relevant invoices invalid.  In accordance with the provisions of Annex XVII, the contractor (or leader in case of a joint tender) submits  invoices in electronic format if the conditions regarding electronic signature specified by Directive  2006/112/EC on VAT are fulfilled, i.e. using a qualified electronic signature or through electronic data  interchange. Reception of invoices by standard format (pdf) or e-mail is not accepted.  If Article  Error! Reference source not found. refers to a formula governing price revision, then the  provisions set out in this Article apply.  The parties agree that the prices shall be subject to a yearly price revision according to the provisions set out  below.  Price revisions shall always be applicable as of 1$^{st}$ January. For a FWC signed during calendar year N, the  first price revision shall be calculated as set out below during year N+1 and become applicable as of 1$^{st}$  January of year N+2.  However, when a product or service was not included in a FWC from the beginning, but introduced through  an amendment signed in year O, otherwise than as a result of a change request, the first price revision for  that product or service shall be calculated during year O+1 and become applicable as of 1$^{st }$January of year  O+2.  As regards hardware, complex and other than complex hardware, and unless otherwise agreed by the  parties, when a product is introduced into the FWC through an amendment signed in year P as a result of a  change request after the procedure set out in Article II.19.3 below has been initiated, the new product shall  bear the price of the product it replaces, i.e. the non-revised price for the remainder of year P, and the  revised price as of 1$^{st}$ January of year P+1.  Between 1$^{st}$ July and 30$^{th}$ September every year, the contracting authority sends to the contractor an initial  written notification informing the latter about the result of the calculation of the yearly price revision.  The contractor has 30 days following the date of receipt to submit observations against the result of the  calculation made by the contracting authority. In absence thereof, the initial written notification sent by the  contracting authority acquires the status of an amendment with full legal effect and enters into force the day  after the time limit for submitting observations has elapsed.  Should the contractor formulate any observations on the correctness of the calculation made pursuant to  paragraph 1 above within the deadline set out in paragraph 2 above, the contracting authority shall carefully  consider them.  29 