Don't panic.
Within the Dispute, give the number of the PP payment.
It is possible that the seller has either a lapsed credit card filed with PP, or if he is very new, gave an incorrect creditcard number or email address.
Let the seller straighten it out.
You have up to 45 days from purchase to Dispute if your seller does not ship or refund. Mark Day 42 (for safety) on your calendar for reference.