Automatic email

Are you talking about a link on your site that people use to pay you through or are you talking about when someone actually goes to paypal? if it is the latter of the two, you would have to write a script that would automatically connect to PayPal every so many minutes and check if the transactions have changed and then kick off and e-mail.

What is this going to be used for?

Are you sending yourself your own password?

I'm not sure I understand you either.
If you want to accept payments by paypal on your site look at the "3 powerful solutions" outlined in https://www.paypal.com/uk/cgi-bin/webscr?cmd=_merchant.  Buy Now Buttons are probably want you need.
HTH

The way you setup Buy Now buttons, you don't need users to have or need any password other than their own Paypal one!
You should also update you paypal account to a business one if you haven't already done this.
The default is that when you receive a new paypal payment you will get emailed, but it's optional.

So you are managing all the code for processing the payments and sending the information to PayPal? I'm sorry I haven't worked a whole lot with PayPal, but I'm sure if I can understand your predicament a little better I'll be able to help you.

No, No,
They buy through the buy now button, and then I need something to send an email to them once I recieve a payment. The email will contain a download password for the ebook product.

Oh, couldn't you use Outlook, setup a rule of some kind

They will get an email anyway from Paypal I think so maybe you don't need to do anything???!!!

Okay, so what you would probably want to do is write a script that will either:

Scan your e-mail and send an e-mail off when you have recieved payment. It would have to parse the e-mail and look for keywords.

or

Log into your paypal account find any new, approved transactions and the e-mail address for them and then send the e-mail.

So what script would I use that would send of an email when I recieve a payment?