I don't know if you can revise your listing and add an international postage option if you've now received a bid, but try it and see.
Go back into your listing and tick the international box in the postage section and add a postage price and method for the US. You'll also need to complete the country of origin item specific too.
Bear in mind that there's now a tariff in place, so when you send your item out from the Post Office you'll be paying 10% of the item price plus a £1.50 admin fee.