All you can do is to contact eBay Customer Services, direct them to the message the buyer sent about a delay in payment and your acceptance. Forgetting there was an cancellation present in your site preferences for a non payment strike after ** days.
Please can they remove the non payment strike on the buyer's account, you can but ask.
I do recommend though to contact CS only very first thing in the morning when the lines first open at 8 a.m. as there's more chance of Dublin answering.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
As said, I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@whaleyboy0