Hi,
Displaying the button in the cart module is indeed the solution.
Then, you'll want to add some custom CSS code on your website to hide the parts of the module that are displayed for that module.
For example, to hide the total price you can do like that:
#hikashop_cart_module .hikashop_cart_module_product_total_value ,#hikashop_cart_module .hikashop_cart_module_product_total_title{ display:none; }
And to have it only for the module in a certain position, just change hikashop_cart_module by the id of the div of that position in your Joomla template (you might need to ask this to your template provider).