With reproducible builds like Signal does you can be sure the app you've downloaded matches the source code that's been audited:
https://github.com/signalapp/Signal-Android/blob/main/reprod...
31 karma · joined April 4, 2023
https://github.com/signalapp/Signal-Android/blob/main/reprod...
They should, but the entire internal metering & bill generation pipeline isn't setup to support that. You can configure billing alerts but these will only refresh on updates to your estimated bill (typically re-computed every 8hrs). And since service teams are responsible for posting metering records to commerce platform their implementation may vary.
TLDR: AWS Commerce Platform is a mess.
Source: Ex-AWS SDE
UPDATE: Looks to be more wide spread, also including Apple Music, iTunes, Podcasts.