Yeah, it's cutting it close this late.
I mailed mine in, the county received it weeks ago.
I don't understand why anybody would wait this long, it's not like there's an actual decision at the national level this year. Just mark Biden, then any other Democrats on your local ballot, decide on any local initiatives, put it in the envelope, sign the envelope, mail the thing in. Took about 10 minutes, including double checking the instructions to make sure there aren't any gotchas on where to sign.