Google Checkout subscription: How to identify the user

We are using Google Checkout - google handled subscriptions. In our html form we are using merchant-item-id parameter to store the subscription plan id and user id, so when Google Checkout send us back the notification for the new orders (merchant-item-id is there) we know what user for what plan to charge. So far this works perfect. But now, when a month i开发者_JS百科s passed, and Google Checkout start creating the reoccurring orders, there is no merchant-item-id parameter in the notifications they send. So we don't know what user for what plan is charged.

What should we use as user identifier, so we can handle properly the subscription on our site?

Any ideas?

Btw. I know about the "buyer-id" parameter which is send with each new order notification, but that will not work for us, because it is possible that the same google buyer is paying for several of our users accounts.

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I would try putting some unique info in "shopping-cart.items.item-1.merchant-private-item-data" to something unique that I can identify the subscription by.

• Nikolaj