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Sounds like a scam. Especially before even receiving payment! I mean, how would you know that you were overpaid this $150? I would contact PayPal to ask if (a) there was even a payment sent to you, (b) if so, whether the customer was charged an extra $150.
The transaction sounds highly suspicious and I'd just tell the person, "Sorry, I don't see payment, so I'm calling PayPal customer service to confirm." They ought to cease emailing you at this point because the jig is up. Know that when someone really paid, you'll see the funds and a record of payment in your account.
Sometimes, a payment may be stopped at the sender account for review by PayPal and you won't receive it until PayPal finishes reviewing it which takes up to 72 hours. Even still, you shouldn't ship out until you see funds in your account, even if the payment is subject to PayPal's 21 days funds availability hold. And what's an 'ITCN"? Doesn't sound like it has anything to do with receiving a PayPal payment.
When you receive a PayPal payment, PayPal sends a very basic payment confirmation email: who paid, how much, for what, transaction ID and shipping address where required and not much else, certainly not all the stuff you said PayPal supposedly sent. Check the email header with your email client to see if it's a fake email address that sent it to you by comparing it against REAL PayPal payment confirmations you've previously received.
I believe the point of the scam is to trick you into paying this $150 as a refund. Scammers don't have to scam much money. Some count on the number of victims to gain the volume.
Emails can be faked and PayPal does not send out these types of emails discussing transactions through your email inbox. They do it through the message center within your account and just email you that you received a response and that you have to log in to your PayPal account to read it.
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