California Registration Issue

I don't know if anyone has had this happen to them but what a bending over. I had to renew my truck registration due at the end of April. My renewal notice says return by mail only. I sent them a check on April 4 from the post office. I have been waiting for my tags and drove my other vehicle around so kind of forgot about it. My wife says, hey Calif hasn't cashed the registration check yet. Of course now its after the due date. I went on their website to check, and of course now it says I need to pay the late fees of $41. I can't prove that it was mailed so I'm hosed. Why it would say by mail only is beyond me. We pay online all the time. So, don't let this happen to you if you get this type of notice. Go waste your day at the nearest office and pay on the spot.