So pay.
If the seller has not opened and won an Unpaid Item Dispute, you have all the time he will allow you to pay. Some sellers here post that they allow upwards of 30 days before getting antsy about payment, but sellers can open an UID in as little as two days.
If it has been over a week, contact the seller, apologize and note that you have now made the payment.
And be sure that your PP account is backed by a credit card so that the payment goes to the seller immediately. If you back the account only with a bank balance, there canbe a delay of another 10 days or so, before the e-cheque clears and he can print the shipping label.