If its marked as completed in your Paypal account then you have paid. The fact its pending on the SELLERS end is between them and paypal as they are new sellers. Funds are held for 21 days BUT released more quickly if they provide a tracking number (delivery + 1 day).
Even if the funds were withdrawn to their bank account they would still have to refund if they lost a dispute. The hold is to protect PAYPAL from new sellers that don't refund so Paypal has to and then has the hassle of getting reimbursed from the seller.
Tell them that if they refuse to send then you would have to file a dispute for 'non receipt of item' and that would impact on their paypal account as they are such new sellers and the time the 'new seller hold' lasts would be extended OR they may not be allowed to sell at all.