Update: I did *finally* get through to customer service. There is a procedure where you can go into Turbotax Deluxe 2018, go to the part where you try to pay by credit card, a window pops up, and an obscure "click here if you've been having problems" link appears that takes you to a set of instructions involving calling customer service and providing a serial number to them. They then use this to find your failed credit card transaction, manually fix it/run it through again, then they provide you with a confirmation number you copy/paste into the software, letting the e-file transaction to go forward. Using this procedure, I was able to e-file.
I am satisfied with the outcome and will use Turbotax's services in the future.
