Hi Tan,
Thanks for notifying this issue as we have already resolved it and will release it in our upcoming version.
But as of now, you can use this patch to resolve you issue.
Please replace the patch file (i.e. order.php inside patch.zip) on the given path mentioned below.
ROOT_FOLDER/plugins/sellacious/order/order.php
And let us know if the issue still persists.
Thanks