@SMAlset , I was on the same boat as you - waiting for the M3 invitation letter/email. Guess what? I logged into the Tesla owners page and clicked on our model 3 page, and bang, the front page was the invitation to configure our M3. I went back and checked our emails, nope, definitely no email from Tesla for M3 configuration invitation.
So, my recommendation to all those wondering why they haven;t got an invite yet - login.
BTW, wife decided to wait for the standard battery... so a couple of more months of waiting for us (hopefully). The good news is, even after we selected the standard version, the choice to configure the long-range version continues to exist - so in case we get up one morning and change our mind, we could get a M3 in 4 weeks (that is the wait time they are now projecting between ordering and delivery).
Good luck!