Ubercart

There is a setting for your store, or in shipping quotes, that sets the default units for weight and length. If you set that to grams, then the weightquote module will use that instead of pounds.

For weight ranges, you need to make a lot of flatrate quote methods. For each weight bracket, you will need a flatrate method and then give its Workflow-ng configuration a condition or two that check the total order weight. The labels that the customer sees should be the same for each method, something like "Standard shipping". That should prevent confusion if they see a quote and then change their order. In your example, the first method would check that the weight is less than or equal to 100g, the second greater than 100g AND less than or equal to 500g, and so on.

To add more options, just make more flatrate methods, but give them different labels and different conditions. For a 600g order, you should see the third "standard" option for $12, and the first "seamail" option for $1.

With this many options and shipping quotes and conditions, it's easy to get confused, so keep a checklist or something to make sure that you cover all of your bases.