If you select the os0 and os1 fields you can see the values for them in the property inspector. If you use the lightning bolt icon here you can set the value to any of your bindings. You should expand the eCart bindings and choose the corresponding columns. Here is an example of what the size would look like:
<input type="hidden" name="on0_<?php echo $eCart1_Index+1; ?>" id="on0_<?php echo $eCart1_Index+1; ?>" value="size" />
<input type="hidden" name="os0_<?php echo $eCart1_Index+1; ?>" id="os0_<?php echo $eCart1_Index+1; ?>" value="<?php echo $eCart1->DisplayInfo("size"); ?>" />