Thanks everyone for the information on mailing, and mailing with the clear envelopes. Unfortunately, I've had trouble with my local post office. Also, there is almost always a language barrier and communication is not clear. I just had to deal with a post office issue twice and everything was clear as mud by the time we got my issues 'resolved.'
I've decided to put mine in regular envelopes rather than take a chance that something will go wrong. I don't want to risk my post card not getting to my partner.