Hi kiwijock,
I’m not sure how payment at the post office works, but I did find this:
https://www.ctt.pt/financas-e-pagamentos/portagens/metodos-de-pagamento/pos-pago.html
If I’ve read it correctly, you have 5 working days to pay at a post office, starting from 48 hours after the journey. I guess they trace it using the vehicle registration number.
There’s also something called EASYTOLL and TOLLSERVICE, but I have no idea how they work, and I doubt very much that they are at all easy! LOL