Paying for multiple items (at once) via paypal

See this sample and make the changes to yours accordingly. Basically add underscore to item name before number and give unique name to amount also with underscore and number.

You have to give amount to each item based on your carts totals.

<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_cart">
<input type="hidden" name="upload" value="1">
<input type="hidden" name="business" value="[email protected]">
<input type="hidden" name="currency_code" value="USD">

<input type="hidden" name="item_name_1" value="beach ball">
<input type="hidden" name="amount_1" value="15">

<input type="hidden" name="item_name_2" value="towel">
<input type="hidden" name="amount_2" value="20">

<input type="image" src="http://www.paypal.com/en_US/i/btn/x-click-but01.gif" name="submit" alt="Make payments with PayPal - it's fast, free and secure!">
</form>

Specify Quantity(optional)

<input type="hidden" name="quantity_1" value="1">

for more reference about cart: Paypal Cart