I send cards in plain white envelopes all the time. I've never had any issues. I receive cards sent in plain white envelopes all the time. I've never had any issues.
Above is my first "issue." Someone in Ottawa tried to send this. It didn't quite make it. It's not obvious from the scan, but in person it's very obvious that what ever was inside the envelope did not make it. I went ahead and added this to my map, even though no cards arrived. The effort was made to send cards, and thats good enough to go up on the map.
Thank you for the attempt Mr. Cook. Our USPS wants you to know that they care and are very sorry. Just don't expect a refund on your stamps.