If you're using a carrier like UPS, within the UPS settings in Stores --> Configuration --> Sales --> Shipping Methods, you can select the "Free Method", making Ground free. You can set a Free Shipping Threshold for UPS in this same area, making this only apply if the subtotal is $150+.
The same is true of FedEx, USPS, and DHL.
If you're not using one of shipping methods, you still have options.
If you're using table rates, you can use Price vs. Destination, and set it up so that Ground shipping over $150 (for the countries you set) is free.
Lastly, you can use the Free Shipping method. You can name this Free Ground Shipping so that shoppers understand that it's ground, and set the minimum order amount to $150. If a shopper wants expedited shipping, they'll need to select a different shipping option that's displayed. (Note: You can also use Shopping Cart Price Rules to further impact when this method is offered)
There are extensions that offer other solutions, but hopefully one of these native solutions will meet your goals.
Best of luck!
