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With Ubercart and Simplenews installed, using Google Checkout results in an error in system log messages, "Attempt to unsubscribe from non existing mailing list ID 0"
Tracked it down to uc_google_checkout.module doing this:
simplenews_unsubscribe_user($order->primary_email,variable_get('uc_google_checkout_simplenews_tid', 0), FALSE);
The problem is nothing is setup to put the variable uc_google_checkout_simplenews_tid in the database or to change it.
I worked around the problem by inserting the correct row into the variables table, setting uc_google_checkout_simplenews_tid to 1. Checkout will now subscribe/unsubscribe from the newsgroup based on "Allow marketing emails" check box when paying at the Google page.