This help page offers our general recommendations for pasting E-junkie buttons into a SquareSpace site:
http://www.e-junkie.com/ej/help.sites.squarespace.htm
To do what you described, as far as I can tell, you would need to use the Code Block tool to insert your entire block of text as raw HTML including the text link. To make that link, you would start with your standard button code from Seller Admin and replace the <img ...etc.> tag with whatever text you want to be linked -- e.g., here's a dummy paragraph including a text link to the cart, such as you might use in a Code Block:
<p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt <a href="https://www.e-junkie.com/ecom/gb.php?c=cart&ejc=2&cl=XXXXXX&i=YYYYYY" target="ej
ejc" class="ec_ejc_thkbx" onclick="return EJEJC_lc(this);">click here to purchase</a> ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.</p>