The PPSCoP states that the discount period is from the issue date of the PCN. If the operator wants to extend that to 14 days from the date the notice is "given", that is their prerogative. It cannot be any less than 14 days from the issue date.
The PPSCoP states in section 8.1.2(e) Note 2: "A notice sent by post is to be presumed, unless the contrary is proved, to have been delivered on the second working day after the day on which it is posted; and for this purpose, “working day” means any day other than a Saturday, Sunday or a public holiday in England and Wales. Therefore, parking operators must retain a record of the date of posting of a notice, not simply of that notice having been generated (e.g. the date that any third-party Mail Consolidator actually put it in the postal system.)" (My emphasis)
So, the operator is required under the PPSCoP to retain an evidential record of the date the notice was physically put into the postal system. Whilst most will not have this evidence and instead try to rely on a "certificate of posting" generated by their own internal system that links to the third-party Mail Consolidator which will only show the date and time that the notice was sent through hybrid mail system to the consolidator. It is not a "proof of posting certificate" or evidence of any sort tat is acceptable.