Thanks, everyone for some excellent advice.
I particularly like Don's suggestion of mailing an old fashioned cheque. Unfortunately, as I am in Canada and the seller is in the US it would not be practical in this case.
I appreciate johnsim03's contention that the seller sets the conditions of a sale, and as I have now been refunded in full, the onus should be on me to return the stamps.
In general, I would agree, however, in this case, I believe that it is not so straightforward.
When something is offered for sale, once a buyer has paid for it, it becomes a contract. If the seller misrepresented the item being sold, either deliberately or in error, then I believe that the seller has broken the contract.
By attempting to contact the seller three times over a period of three weeks, I went out of my way to try and resurrect the contract. Given that the seller completely ignored my attempts at resurrecting the contract I believe that the seller has forfeited the right to set the terms of the contract.
It should also be noted that the seller did not voluntarily give the refund, the refund was initiated by
ebay.
What I propose doing is to contact the seller to get their email address which I would need for a PayPal payment.
At the same time, I will give him two options:
1. I can return all three stamps at his expense, I don't see why I should be out of pocket for even the few dollars postage given his complete disregard for a paying customer.
2. Or I can send him a payment for the two stamps that I want, less the cost of returning the third stamp to him.
Once the transaction is finalised I intend to leave the negative feedback that I believe he deserves - not for the initial misidentified mistake, but instead for an appalling lack of consideration for his customers.
If however, I do not hear back from him, and I suspect that I won't, then I will doff my hat to rogdcam's little devil

and go to bed at night with a clear conscious in the belief that I have gone beyond what is reasonable in trying to ensure an equitable outcome for both parties. In this case, I won't leave any feedback at all
This is not the first time that I have received a "not as described lot". In most cases, it has been a genuine error on the part of the sellers who have immediately issued a refund, and I in turn have had no quibble with returning the item at my expense.
Clive