Why is the PayPal payment I received under review?

PayPal reviews all transactions to help sellers avoid risk. If we think that a payment might be risky or fraudulent, we'll email you that the payment is under review, and ask you to wait until our review is complete before shipping. This review can take up to 24 hours.
 
You can also check the progress of your payments on the Activity page. The payment status of your transaction remains “Pending” until we complete our review.

Payments that clear Payment Review are automatically eligible for Seller Protection as long as the transaction follows our guidelines.