Merci Laurent!
Attached the letter that made be curious about this steamship service.
Official letter (exempt for territorial tax due to the S.N.R. and mark from
Conselho de Saude Pública) sent from Lisbon, Portugal on January 29, 1857 to Stockholm. Transit mark from Badajoz in blue where it was placed in closed bag and dispatched via France and Belgium. “VERVIERS / 8 2 1 / COELN” trilinear mark in red and Boxed mark SPANIEN also in red.
In Prussia, written in the back 8 Sgr for an unpaid letter coming from Portugal via Aachen up to 7,5g according to the postal convention between Spain and Prussia (4 Sgr for Spain, 3½ Sgr for France plus ½ for Belgium).
Then eventually, 3 Sgr added for transit in Prussia for a letter going to Sockholm, according to postal convention between Sweden and Prussia, resulting in 11 being written in the front.
Circular mark from Hamburg (8.2) and K.D.O.P.A. / Hamburg (9.2). From Swedish Hamburg post office likely by steam service to Stockholm. At the destination 1 Riksdaler written in the front.
I am still looking but I think 1 Rds was equivalent to 48 Skilling. And still couldn't find the accounting to reach the 1Rds ... and don't know yet how many Skillings would be 11 Sgr.