You need to know what to optimise which means you need to know what you are spending on.

I did say it applies IFF and only IFF you choose to use these services, and if you have chosen to use these services you have presumably decided they are good value for money. If not, why are they using AWS.

Of course the complexity and extra cost of managing the billing is something that someone who has chosen to use AWS has already factored in, right?

The alternative is to not use AWS.