Rule for sending email when order exceeds a defined weight
After some fiddling I've found the solution myself:

    { "rules_notice_overweight" : {
    "LABEL" : "Overweight",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "REQUIRES" : [ "commerce_physical", "rules", "commerce_shipping" ],
    "ON" : { "commerce_shipping_calculate_rate" : [] },
    "IF" : [
      { "commerce_physical_rules_order_weight_comparison" : {
          "commerce_order" : [ "commerce-line-item:order" ],
          "operator" : "\u003E",
          "value" : "10",
          "unit" : "kg"
        }
      }
    ],
    "DO" : [ { "drupal_message" : { "message" : "Total weight is over 10kg" } } ]
  }
}


The problem in my first code example was the trigger "commerce_shipping_collect_rates". It has to be "commerce_shipping_calculate_rate" instead. I hope someone can benefit from that.