I'm having quite the time with Uber support right now as well. Similar situation, Uber Cash not being applied, even though I'm fairly certain I selected it, with the additional charges to go to my credit card. Asked about it, and they won't apply the credit because it's not enough to cover the trip. I found a cite in their help documentation which explicitly says that "you may choose to pay the remaining balance for the cost of the ride using your preferred payment method". Asked about that specific line, and got another support message repeating the info in the previous one, but now this time the've changed the badge at the end to "no further response required".

https://help.uber.com/riders/article...2-2acdd7990205

I mean, I'm sure it saves money to make the credit difficult to use, but it's not a good look for AmEx (I'm sure Uber doesn't care how they're perceived).