Can I charge less shipping on bigger orders?

Yes. In the Shipping Zone plugin, use a Price-based (brackets) method:

1. Go to Plugins → Shipping Zone and click Edit on the zone.
2. Click + Add method and set Type to Price-based (brackets).
3. Click + Add bracket for each range of cart value and fill in Min amount, Max amount and Cost.
4. For the top bracket, leave Max empty and tick Free.
5. Add a Label and click Save zone.

Wcart picks the bracket that matches the cart total. Make the ranges continuous so no cart value falls between brackets, because an unmatched value is charged nothing.