Is this happening on just this one order and others go through? If so I wouldn't try to make any changes as it sounds more like customer behaviour rather than a setting rrror.
I am not sure, this customer is know to me so he cared to let me know.
Otherwise, probably wouldn’t have bothered as PayPal sent him a transaction successful email and then he also received a email from our website stating that the order has been received.
I don’t know how many other customer might receive the same message.
It could have been a server glitch or something the customer did like refreshing or something along those lines. I would run through a dummy order yourself tio be sure though.