How do I access the details of my monthly invoice?

That's a good question, @gracaboy. They say it is about confidentiality reasons...
In case there are more than one mobile service associated with the same package / invoice, customers should not be aware of communications details of their spouses, children, etc..
However, if a customer has only one mobile service, it is assumed that he will receive the information himself… but they treat all the same…

You can consult the communications details in your Customer Area, just have to register the services contracted in there.
To view the communications details for a given period, up to a maximum of 6 months:
Customer Area > Os meus serviços > Gerir Serviço (base package) > Telefone > Detalhe de comunicações > (set period to search)
Or
Customer Area > Os meus serviços > Gerir Serviço (base package or mobile service) > Telemóvel > Detalhe de comunicações > (*) (set period to search)
(* You have to request the authentication code to be sent to your mobile phone - "Receber o codigo de confirmaçao")

To obtain a specific invoice plus an appendix with communications details for that period:
Customer Area > Os meus serviços > Gerir Serviço (base Package) > Faturas e pagamentos > Faturas > Últimas faturas > Obter Documentos > Descarregar detalhe de comunicações (PDF)

Your last invoice may not have yet the documents available for download, usually they will be available at the end of the invoice cycle and not at the invoice issuance or delivery dates.