Google Analytics ecommerce tracking requires that a client side javascript is run the user's browser when they complete an order. This happens in Kartris on the CheckoutComplete.aspx page right after returning from the payment gateway (see 3. above).

It cannot be done on the callback because that is called by PayPal, not the customer's browser. Unfortunately PayPal puts in a 5-second delay after a completed payment before returning the customer to the store. This inevitably results in some customers dropping out after a successful payment but before they have been redirected back to your site, and hence not being logged by Google Analytics as completed sales. There is no workaround for this; the Google Analytics code can only run on the user's browser; it must run after the payment is complete (otherwise you will log sales as successful that don't complete payment) and PayPal apparently has no way to reduce or eliminate the 5-second delay). This is not a Kartris issue alone; it is an issue with all carts that support Google Analytics when they are set up to use PayPal Web Site Payments Standard.