Well, here's what I did to resolve this:
<input type="hidden" name="on0_<?php echo $pkCart_Index+1; ?>" id="on0_<?php echo $pkCart_Index+1; ?>" value="<?php $size = $pkCart->DisplayInfo("Size"); if ($size != "") {?>Size<?php }?>"/>
<input type="hidden" name="os0_<?php echo $pkCart_Index+1; ?>" id="os0_<?php echo $pkCart_Index+1; ?>" value="<?php if ($size != "") {echo $pkCart->DisplayInfo("Size"); }?>"/>
I would love to see a better way to do this, but at least this works.