Holy Moly it worked.
Thank you!
Okay, so it's up and running with the PayPal IPN module installed. Let me run thru everything I learned for the benefit of those watching at home and those furiously searching this forum for the same answers.
Uploading the files via Dreamweaver didn't seem to work. So I started over with Terra's latest version (1.3) and uploaded the two "paypal_ipn.php" files directly thru OSC Admin>File Manager. I recreated the folder structure for the "ext" batch one folder at a time and then uploaded the "ipn.php" file too. I know it's inconceivable, but I don't have an FTP program. I've been fine up til now using Dreamweaver! Anyway, Voila! I go to Admin>Modules and "PayPal (Credit Card/Debit)" is now there (highlighting it shows "IPN" in the righthand menu). I set it up according to Terra's install guide and it's good to go. After running a few test transactions everything seems to be working, granted the known issues outlined in Terra's install instructions. Bottom Line: If a customer does NOT click "Return to Merchant" or "Complete Order Confirmation" after their PayPal transaction I nonetheless have a copy of their order details in Admin>Customers>Orders, with the order status marked "Preparing [PayPal IPN]". To determine if these orders are valid, I must manually compare my list of OSC orders marked "Preparing [PayPal IPN]" with the list of completed transactions in my PayPal account. And I have to manually update the stock. A bit of a pain. Actually an enormous pain if you're planning on processing a lot of orders... But it works?And hey, I'm not complaining. Free, useable open-source code with extensive support forums? better than a stick in the eye. The only danger is that it makes any idiot with an internet connection (me) think he can code an ecommerce site?The other drawback not mentioned in Terra's known issue list is that any comments the customer wrote during Checkout are lost (if the customer doesn't return to my site from PayPal).
Does all that make sense?
All the above are issues only if the customer DOES NOT return to my site after PayPal transaction. To help alleviate this I put in a line of text on the checkout_confirmation page that says something like "Once you have completed your PayPal transaction, be sure to click Return To Merchant" with a link to a page displaying further explanation. Sort of ghetto, but I'm small time so I think I can get away with it. It's better than one of those Under Construction animated gifs.
Here's one other issue I noticed. Maybe it's been addressed somewhere else?
From the /checkout_confirmation.php page a customer can manually redirect the URL to /checkout_process.php and fool OSC into thinking they bought whatever products were in their cart, without paying for them. When this is done OSC sends an order receipt to this "customer" and deducts the product stock, and the order appears in the Admin (marked "Preparing [PayPal IPN]"). It doesn't fool PayPal of course, but this loophole could create quite a mess for store owners to sort out. Any solutions on that one?
Again, thanks! really appreciate the help. sorry for the long post but I hope it'll help some other poor lost soul.
c.