By avoiding multiple visits by citizens to obtain services. This article (in Portuguese) proposes a methodology to calculate the reduction potential of these visits in local government by appling interoperability: https://www.researchgate.net/publication/261488092_Analysis_of_the_potential_for_organizational_interoperability_improvement_in_local_government
Conference Paper Analysis of the potential for organizational interoperabilit...
We can get an estimation of how many citizen visits are avoid by delivering services online. The most difficult part is to estimate what are the average carbon footprint savings associated with that. I imagine this depends greatly on the specif context.