Support for transaction receipt logging is an improvement in 5.4.3 but I am unable to find any info on what this feature does and how we would use it.
This will mostly be a feature for people using Raw Data Export and Unity IAP. We modified the format of the transaction event to have the ability to see which store the transaction was called from. So if you have an app on Google Play and Amazon stores, your Raw Data transaction events will now show which store the transaction is from.
This affects Unity IAP, since the Analytics.Transaction events are used internally.