You could create an insurance "product" in Seller Admin and then display an Add to Cart button/link for that item within the cart itself. On every page where you want the insurance offer to be shown, just add the following lines to your View Cart code, just before the "// -->" line in the standard View Cart code you obtained from Seller Admin:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
jQuery("#tdPmnt").attr("innerHTML",
"<a target=ej_ejc class=ec_ejc_thkbx onClick='javascript:return EJEJC_lc(this);'
href='https://www.e-junkie.com/ecom/gb.php?c=cart\&i=XXXXXX\&cl=101026\&ejc=2'>
Click here to add shipping insurance to your order</a>");
}
Note that you should replace XXXXXX with the Item Number of your insurance product.