you need to have a value for your item variables. If you pass a blank value, it would be ignored by PayPal (all other items after item 2 as well). Instead of passing a blank value, you can use 0 with "amount_2" variable (i.e. if checkbox is not checked, amount_2="0").
Here's an HTML example:
<input type="hidden" name="amount_2" value="0">
<input type="checkbox" name="amount_2" value="1.00">