This is a new ' feature' eBay introduced to stop the number of non payments sellers were experiencing, when a buyer has accepted an offer.
Now, a method of payment has to be given and payment is then made immediately.
It's a good idea on paper, but does have its flaws. You cannot select the payment method you'd prefer to use, combined postage cannot be offered, neither can the Click and Collect option, to name but a few.
@waff80