If your main objective is to add multiple items to the buyer's cart at the same time, we'd recommend using our cart's built-in EJEJC_multiAdd() function described here:
Note that function is part of our box.js cart-overlay script that displays the cart "inside" your own page, so make sure at least the SCRIPT portions of your View Cart code in the page.
All cart parameters can be passed either as GET parameters on the cart URL or as POST parameters in a form -- e.g., the 'i' value for item number is normally provided in our button code as a GET parameter on the item's cart button URL (&i=123456), but you can also pass it as a POST parameter in a form field (<input type="hidden" name="i" value="123456">).
If you want to pass those parameters to the overlay-style cart that appears "inside" your own page, you will need to have your script pass the desired URL and GET/POST params to function EJEJC_lc() of our box.js cart-overlay script.
If your cart is displaying in a separate window/tab, your scripting can target the GET/POST request to the name of that window/tab; we'd recommend the target name "ej_ejc", as that's what our standard cart button code uses.
If you require further assistance, we'd need a more detailed explanation of what you're trying to do and how, along with a link to your page where you're setting it up; if you'd rather not share those details here, feel free to email us:
P.S. Google recently retired the custom site search method we'd been using, so we've temporarily removed our site's search function until we can build a new one using Google's latest method.