Just thought I'd add my experience here for the group:
I ordered a 8.16kW system, with 2x powerwalls, on 2SEP2020. I live in the SF bay area in CA.
My final invoice was delivered last night.
I logged on and found the ACH-only (plaid - ironic) option only.
I emailed my solar adviser asking if I could pay by CC.
Checked here and on Reddit and found some people were able to still pay with CC by calling in.
Solar adviser emailed back saying they no longer accept CC payments; I replied that this was disappointing and misleading (no communication of the change in policy).
This morning I called the number listed in this thread. Got an agent who seemed willing to help, but was having computer issues. She said she'd call me back after a reboot of her computer.
She called back, but couldn't hear me, and hung up. I called the number again, but got someone else, this guy said if my only option shown in my account was online payment, then I could only pay with ACH. I told him I was very unhappy about that and hung up.
A few minutes later the original agent called back, and processed my payment with my CC no problem, no questions. I didn't ask about service fees, but there weren't any.
I did not get the impression that the two communicated. I think the first agent just called back because she said she would, and that she intended to process it all along, and the other one wouldn't.
I now have a pending charge on my Amex. Tesla account has not updated yet though. This all happened this morning.
TL/DR: if you really want to use your CC, keep trying. I also indicated to the first agent that when I placed the order back in Sept, I was under the impression that I could pay with CC. In the end, it worked out.