In case of loss or delay, the EPO accepts evidence of mailing a document only if it was mailed via the postal authorities or if one of the following postal service providers was used instead: Chronopost, DHL, Federal Express, flexpress, TNT, SkyNet, UPS or Transworld. As evidence, confirmation of registration by the post office or confirmation of receipt by the postal service provider must be provided at the request of the EPO. 

Please note that Rule 82 PCT excusing a delay in the post and Rule 82quater PCT excusing a delay due to force majeure are not applicable to the priority period, because these provisions may only be applied in respect of time limits fixed in the PCT. However, under strict conditions a right of priority may be restored (see points 133 ff). It is, therefore, recommended that an application be filed as early as possible.