Keeping customers from choosing between “free shipping” and shipping cost

I’m using the plugin “Postmen WooCommerce Shipping”, and once I posted my topic on the plugin’s forum, I saw that nobody else’s questions have been answered on the forum for one and a half years, which was when the first topic was created. So I’m hoping I could get some answers on this forum.

I think I’ve pretty much figured out this plugin, except that I don’t want my customers to be able to choose between “free shipping” and my designated price of shipping. They’ll most likely choose the free shipping because nobody wants to pay more than they have to.
So how can I prevent that from happening?