In order to ensure that PayPal calls back your Kartris site, to notify it that a payment for a particular order was made so that confirmation mails can be sent and the order can be tagged as 'paid', you need to set up a few things within your PayPal account.

1. Login to PayPal and go to My Account > Profile. Then click to view My Selling Preferences (left hand side) and then a bit over half way down, click the 'update' link by 'Instant payment notifications'.
2. The Notification URL should be http://www.mysite.xyz/Callback-PayPal.aspx. The 'Receive IPN messages' radio button should be checked.
3. Back on the My Selling Preferences page, click the 'update' link by 'Website preferences'. Turn 'Auto Return' on, and enter http://www.mysite.xyz/CheckoutComplete.aspx as the 'Return URL'. This will redirect the user back to your site after they complete a payment on PayPal. But it is important to note that the order itself is called back immediately after payment by PayPal's server calling the Callback.aspx page of your site (see 2. immediately above).
4. Set 'Payment Data Transfer' off - we don't need this as we send info via the IPN step in 2. above.

You may find that orders where data submitted contains non-English characters (such as French or German accented characters) generate an INVALID response at PayPal. To fix this:

1. Log into PayPal
2. Go to My Selling Preferences (see above)
3. Click 'Paypal Button Language Encoding' link at foot of page, and then click the 'more options' button
4. Set 'Encoding' to 'UTF-8' 
5. Set 'Do you want to use the same encoding for data sent from paypal to you?' to 'Yes'.
6. Click 'Save' 

We advise doing this even if you're in the US or UK, as it's quite possible you'll have customers with names and/or addresses either domestically or overseas that contain non-English accented characters.