I dont see a problem at line 408 - 418. where I see a problem is at line 431 - 432:
<input type="hidden" name="on0_<?php echo $megan_isaacs_Index+1; ?>" id="on0_<?php echo $megan_isaacs_Index+1; ?>" value="<?php echo $megan_isaacs->DisplayInfo("Description"); ?>" />
<input type="hidden" name="os0_<?php echo $megan_isaacs_Index+1; ?>" id="os0_<?php echo $megan_isaacs_Index+1; ?>" value="<?php echo $megan_isaacs->DisplayInfo("Size"); ?>" />
The on0_<int>, os0_<int> are used to send optional information to paypal. the on0_<int> element is used to define the name of the option, os0_<int> is used to send the value of the option. I think the problem is occurring where you are sending the Cart Description ans the option name. Try changing that to:
<input type="hidden" name="on0_<?php echo $megan_isaacs_Index+1; ?>" id="on0_<?php echo $megan_isaacs_Index+1; ?>" value="Size" />
<input type="hidden" name="os0_<?php echo $megan_isaacs_Index+1; ?>" id="os0_<?php echo $megan_isaacs_Index+1; ?>" value="<?php echo $megan_isaacs->DisplayInfo("Size"); ?>" />