You can offer Priority Shipping to your customers simply by setting the tier for which their order applies to as Priority / Express shipping.
This can be done for any order placed based on it's value or weight.
$0 to $30 - Normal shipping, you charge $7
$30 to $100 - Express shipping, you charge $12
In the example above, the order will automatically default to Express shipping if the order value is equal to or exceeds $30.
It is important to note that the customer will not be given the option to choose from Standard or Express.
In the interest of simplicity for both the store owner and customer multi channel shipping is not currently available using the current shipping methods.
We do however plan to introduce multi channel shipping in future which you'll be able to offer as options to your customers at the checkout.