Is this too late?

The return window has been extended due to the virus.
You need to accept the return request. As it is an ‘Accidental order’ the customer is responsible for the return shipping cost so issue them with an unpaid shipping label via the return request.
If they are aware of this and it is likely to cost a lot to return then they might decide to keep it - guess it depends what it is.
Tell your buyer that it must be returned in the same condition as when sent and once you have received it back safely you will issue the refund. Don’t refund until you have it back.