I've found a conflict with the webhook events as previously there was only my webstore in the mix. Since adding the donation page I had to do a different processing script to handle the donation stuff. Unknowingly to me at the time, the checkout session event triggers both endpoints scripts.

Donations were causing "corrupted" orders, and orders were causing the donation page to update with odd stuff.
I have patched the web store and the donation scripts just to check exactly what events were being triggered (hopefully). Unfortunately I cannot test either until someone does a web store order and donation.
So if something looks broken with the web store checkout or donation page after we posting this message then just let me know... There should not be any problems but knowing my typos...