Thanks for the Costco tip OP! I rebooked with them this morning and dropped another $90 off my cost. Down to $280 (including taxes and fees).
Only thing is (and maybe here's a hint for car renters who might be in my situation): for various reasons, my husband and I choose not to use credit cards. Don't even have any. Personal choice we made years ago after some poor youthful choices. Anyhoo, renting a car is always somewhat of a challenge, but we found that in WDW, if you rent with Alamo and have a round-trip airline ticket, you can pay with a debit card and THEY DON'T REQUIRE A DEPOSIT. (Most companies will let you use a debit but they lock up double the amount of the rental on the card as a deposit.) So when I had originally booked though Alamo in CA (picking up at LAX), it was frustrating me that they didn't extend the same courtesy.
So I called Alamo and asked them if there were any Alamo rental places in the
Disneyland area that didn't require the deposit, like in FL. Lo and behold, the Alamo place on Disneyland Drive (next to the Grand Californian) works just like the Alamo rentals in WDW - you can use a debit card and no deposit is required as long as you show a round-trip ticket.

And I can pick the car up in DL, and return it at LAX.
OK, new decision... how to get from LAX to DL when we arrive to pick up the car. After going through all this to get a lower car rental cost, I really don't want to just smack another $100 shuttle cost onto the top of it. So with some investigation, I discover the Los Angeles Metro. For a grand total of $3 each (and a little of our time, it isn't speedy) we can take the train/bus and it will drop us off at the East Entrance of the DL plaza. A stroll through Downtown Disney and maybe a bite to eat will have us picking up our car and off to our hotel.
YAY! As long as it works as planned I think I have it all figured out.