Google Checkout - XML API associate callback serial number with original order - google-checkout

Question

Via the XML API, how do you associate an Google Checkout callback serial number with the original order?
On the same line - What does the serial number in the "Option B - Submit a Server-to-Server Checkout API Request" section of the XML API doc correspond to (format: serial-number="981283ea-c324-44bb-a10c-fc3b2eba5707")? Does it relate to the serial sent by the callback URL (numeric-only)?

Answer 1

The way I've done this in the past is using the <merchanrt-private-data> tag in the original cart, so something like:
<checkout-shopping-cart xmlns='http://checkout.google.com/schema/2'>
<shopping-cart>
<merchant-private-data>
<merchant-note>[some secret about the cart on my system]</merchant-note>
</merchant-private-data>
<items>
...
</items>
</shopping-cart>
</checkout-shopping-cart>
Then, after Google has called back with a serial number, I use the Notification History API to retrieve the order details, which then includes my private data, something like:
<new-order-notification xmlns="http://checkout.google.com/schema/2" serial-number="[serial number from google]">
<buyer-billing-address>
...
</buyer-billing-address>
<timestamp>...</timestamp>
<google-order-number>...</google-order-number>
<order-summary>
<total-chargeback-amount currency="GBP">...</total-chargeback-amount>
<google-order-number>...</google-order-number>
<total-charge-amount currency="GBP">...</total-charge-amount>
<total-refund-amount currency="GBP">...</total-refund-amount>
<purchase-date>...</purchase-date>
<archived>false</archived>
<shopping-cart>
<merchant-private-data>
<merchant-note>[the secret about the cart from my system]</merchant-note>
</merchant-private-data>
<items>
</items>
</shopping-cart>
<order-adjustment>
...
</order-adjustment>
<promotions />
<buyer-id>...</buyer-id>
<buyer-marketing-preferences>
<email-allowed>false</email-allowed>
</buyer-marketing-preferences>
<buyer-shipping-address>
...
</buyer-shipping-address>
<order-total currency="GBP">...</order-total>
<fulfillment-order-state>NEW</fulfillment-order-state>
<financial-order-state>REVIEWING</financial-order-state>
</order-summary>
<shopping-cart>
<merchant-private-data>
<merchant-note>[the secret about the cart from my system]</merchant-note>
</merchant-private-data>
<items>
</items>
</shopping-cart>
<order-adjustment>
...
</order-adjustment>
<promotions />
<buyer-id>...</buyer-id>
<buyer-marketing-preferences>
<email-allowed>false</email-allowed>
</buyer-marketing-preferences>
<buyer-shipping-address>
...
</buyer-shipping-address>
<order-total currency="GBP">...</order-total>
<fulfillment-order-state>NEW</fulfillment-order-state>
<financial-order-state>REVIEWING</financial-order-state>
</new-order-notification>
I can then use the secret to match the orders up to the details I'd stored in the database previously.