Please explain PayPal Express data flow

Can someone please give me an explanation of the data flow/log file creation process for PayPal Express? We're trying to troubleshoot a sporadic problem in which the order process appears to be interrupted. I don't think it's just paypal, but paypal seems to have a very good logging function so I'm hoping this will help.

For example, I have a customer who tried and failed to place an order. In the paypal log directory, I found three files for SetExpressCheckout for this order, but none of the other log files that are usually created.

We're using 1.3.9h.

Thanks,

John

1. SetExpressCheckout -- tells PayPal about the basic details of the order, and receives a token from PayPal.
2. Customer is redirected to PayPal using the token. The customer logs in and selects their funding method, and returns to your store.
3. Your store sends a GetExpressCheckoutDetails request, to obtain address details about the customer, and create an account in your store if needed.
4. If your customer had not been logged in and thus didn't have a change to select shipping details yet, your customer is then presented with shipping choices, asked to enter any coupons or gift certs on your site, see the calculated taxes, and is then taken to the Checkout Confirmation screen.
5. When the customer clicks Confirm, a DoExpressCheckoutPayment request is sent to PayPal to actually complete the payment and move funds between accounts. When that finishes, your customer sees the checkout-success screen.

Step 4 might happen before step 1 if the customer has an account and was logged in and chose the normal checkout flow (not the orange button on the Shopping Cart or login screen) and thus chose the PayPal radio-button on the checkout-payment screen.

The 3 logs that are generated for each transaction are those highlighted. And they're generated in that order.