It's a setting on the merchant's side, have you contacted the seller or merchant?
Because the system recognizes that the invoice has already been paid, should a second customer try to purchase the same item, he or she will get the following message: "This invoice has already been paid. For more information, please contact the merchant".
This setting can be turned off however, the seller or merchant must do that in their PayPal account, select Profile, under My Selling Tools, Block Payments, select Update, look for Payment Receiving Preferences.