Was this because an email told you that you had to send a tracking number before you saw the money in your account?
If so, get to the post office ASAP and recall the package.
However, if you can actually see the payment in your account, it's not a scam, it's the 21 day hold. On the main transaction page of your Paypal, look for the line where you received the payment. Immediately to the right of the "Shipping" button, there's a separate button with a down arrow on it - that's a separate link. Click on it and enter your tracking number there, then Save the entry.
Once the parcel has been deliered ( which you will see if you input the tracking number into the appropriate box on the website for the carrier), then Paypal will release the funds in about 3 days.