Paypal cart upload multiple items

Hi, I'm building a site for competition entry and I can't ask the user to add each item to the cart, I want to compile all their choices and add it to the cart in one go, while allowing them to see the individual products they have selected, not to aggregate them, before they pay.

When I try and use: <perch:input type="hidden" name="item_name_1" value="Test Item" /> instead of <perch:input type="hidden" name="item_name" value="Test Item" />

PayPal responds with an empty cart ?

Obviously adding more than one item gives the same result.

here is my full ADD TO CART button code:

<perch:form id="_xclick" method="post" action="https://www.paypal.com/cgi-bin/webscr">

    <perch:input type="hidden" name="business" value="<perch:setting id="perch_shop_paypal_email" />" />
    <perch:input type="hidden" name="currency_code" value="EUR" /> 
    <perch:input type="hidden" name="cmd" value="_cart" />
    <perch:input type="hidden" name="upload" value="1" />
    <perch:input type="hidden" name="bn" value="XXX_AddToCart_WPS_IE" />  

<perch:input type="hidden" name="item_name_1" value="Test Item" />
<perch:input type="hidden" name="item_number_1" value="123456" />
<perch:input type="hidden" name="amount_1" value="2.50" />
    <perch:input type="hidden" name="quantity_1" value="2" />

<perch:input type="submit" name="add" value="Add to cart" />

</perch:form>

I wan't to add multiple items e.g. item_name_2, item_name_3 etc... but getting one to work would be an amazing start !

I would be really grateful if anyone could help with this, I've been through the paypal IPN so many times now, it's not funny :-(

Thanks Dylan