The PCN can be sent by post: (
https://www.legislation.gov.uk/ukpga/2012/9/schedule/4)
(4)The notice must be given by—
(a)handing it to the keeper, or leaving it at a current address for service for the keeper, within the relevant period; or
(b)sending it by post to a current address for service for the keeper so that it is delivered to that address within the relevant period.
(5)The relevant period for the purposes of sub-paragraph (4) is the period of 14 days beginning with the day after that on which the specified period of parking ended.
(6)A notice sent by post is to be presumed, unless the contrary is proved, to have been delivered (and so “given” for the purposes of sub-paragraph (4)) 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.
There is no “fine”, this is based on an invoice for payment under the contract the driver entered into by parking and by reading the signs.
You or the registered keeper should search the forum for many other ECP cases and court claims, in which you will see that (a) the claims are for well over £100 including additional fees and (b) the majority of properly defended court claims never end up in court, because it’s not worth the while of the claimant to pay for someone to attend. But we can’t promise anything, only direct you to look and see for yourself.
You appear to have the basis of a defence in that the registered keeper can not be liable for the actions of the driver if the driver has not been identified and if ECP has not complied with PoFA 2012 to transfer the liability to the registered keeper.