There’s a separate but related issue.
The notice that comes with the registration renewal notice states (and has always stated for as long as I can remember) that, once you send in your renewal form and money that you may drive your vehicle on the roads, even though you haven’t actually received your new license plate tag yet. (I forget whether it specifies any one-month time limit on that.)
Thus, you can drive with an expired tag (for one extra month?) providing that you’ve sent in your renewal.
This seems to imply one of two possbilities:
(a) Cops will not try to enforce tags that are expired within one month of their expiration, or
(b) When cop sees a recently-expired tag, he can phone home to ask if the registration has been renewed.
I think if the OP has actually not sent in his renewal papers on time, he loses his case. But if he did send in his papers on time, and the DMV has processed that (as evidenced, perhaps, by the day the check cleared with OP’s bank) then the OP wins his case. But that’s the case where the cop should never have ticketed the car in the first place.
An ambiguous (I think) case could arise if the owner sends in his money in just the last day or two before the deadline, and the registration expired before the DMV receives or processes it. I think the owner is supposed to be covered in that case too. So there must necessarily be some implied “grace period” for that.
Note that CA registration periods run from an exact date to an exact date (for example, March 14 to March 14), yet the tag only specifies the month (March). So from that alone, you get a grace period of several days – from the exact day of expiration to the end of the month – depending on what day of the month it expires.
My car, for example, expires on the 18th of a certain month. So I have 12 days from then to the end of the month, and presumably an extra month before a cop would bother. I think the cop who ticketed the OP’s car was being a dick just for bothering to check the registration (assuming he even did that).