We go through Costco when we go to a location it serves. The free additional driver is a benefit that would normally cost €10 a day. The free additional driver is also specified in the paperwork you receive that contains both the Costco and Enterprise/Alamo confirmation numbers. In the extremely unlikely event you did get charged, I’m more than confident Costco would reimburse you. That’s just the way Costco operates.
Costco does not always offer automatic transmission vehicles, except for the UK, and to my knowledge there’s no way to request one in its site. For me it isn’t an issue. If one isn’t available I’ll drive a manual. On my 2024 Spain trip when I picked up the rental I could have opted for an automatic for an additional €15 a day. I kept the manual. For my upcoming Spain trip, Costco listed automatics for the same price as a manual, so I opted for it.
@kj - one selects the zero deductible coverage at the counter when picking up the rental.