Ok, so.. I'm attaching 2 screenshots here so we can better understand the situation.. There's the Shop Page (which shows all products, and each have its price + name + button), and then when we click on one of the products we go to the Checkout Page (where there's a description and additional information of the product)...
If I proceed to do the way you said, the only button changing is in the Checkout Page (the button says (Choose Flavour), so it doesn't help me, because I need it in the Shop Page instead... Makes sense? Sorry if I'm not explaining better...
Now, squarespace doesn't add a button on the shop page at all.. I've added it through coding (that's why I included the footer injection here).. So what I need in fact is someone to help me with the coding bit..
Thank you again for taking the time to answer!