Generally good experience with National, maybe it is Emerald Executive that gives the extra bit. Never had a problem with our letter from Amex either(even when I wasn't able to print it this last time and had to present it on my phone). Remembering the harrowing bus rides from a decade before, the first time back we rented a car doing the full deal, I didn't have any regrets about it. Not being as bad as I remembered we use credit card coverage and the TPL. We also don't hang out in the cities FWIW. LIR has been easier to deal with than SJO, just do the documentation and go over the car with extra care. Bumpers and the space between the doors and roof in particular.