A seller marked an item as paid before he sent the shipping cost and now ebay won't let me pay.

[silk-backing]

The seller had marked the item as paid and shipped before either was done.  He then sent me the shipping cost through my inbox and there was a tab to pay now.  Every time I try to pay ebay just opens another window with the order details.  Can the seller unmark this item as paid to allow me to pay for it?

[reallynicestamps]

Yes he can.

Just use Contact Seller on the listing to tell him the problem.

As things stand, not only is this frustrating for you, but he is being charged fees on the (nonexistent) payment and risks a Defect for slow shipping.