The 14-day period for making a risk-free* challenge and payment of the discount ends today, 22 Apr(14 days beginning on 9th, not from 9th).
I'd make simple reps that you understood that parking with 2 wheels on the footway was allowed in the road as you have seen cars doing this on a regular basis. If you are wrong then you apologise and would ask the authority to exercise discretion and cancel as this is your first** PCN.
*- you haven't posted the back of the PCN, so I'm assuming that this provision - which is commonplace- is included.
**- if true.
Are you the registered keeper with current DVLA