Now that Bidpay is no more, what is the easiest way to send an online money order?

Check to see if your bank offers online banking. If so, you can add the seller's name and address into the list of "Payees" and issue a check that way. The bank deducts the funds from your account and issues a check to the payee. Then once you've received your item, you simply go back in and delete their name from the Payee list.