Ok, with paypal standard, there will be a hidden form element for the item name that will look like:
<input type="hidden" name="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" id="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" value="<?php echo $WA_Store_Cart->DisplayInfo("Name") ?>" />
you could concatenate the session value to the end of it:
<input type="hidden" name="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" id="item_name_<?php echo $WA_Store_Cart_Index+1; ?>" value="<?php echo $WA_Store_Cart->DisplayInfo("Name") ?> - delivery Time: <?php echo ((isset($_SESSION["deleveryTime"]))?$_SESSION["deleveryTime"]:"") ?>" />