To do the free shipping, use a 'cart rule' (back office, price rules->cart rules) instead.
on informations-tab,
add a descriptive name ("free shipping over 150" or so.
Set a validity period for the voucher (set to distant future if always want to give)
Don't add a code for the voucher, then it will be added automatically tot he cart.
Add how many vouchers you want to give (set to 9999999 to give always),
and how many per customer it can be used (set to 99999, if a customer can get the free shipping every time).
on conditions-tab, Add minimum amount is 150, no need for additional restrictions (then all products, all delivery countries, carriers etc are valid to get the discount.
Set free shipping as action on action-tab
Hope this helps,
pascal.