The car on the sticker looks to me a bit too much like a Flavia Vignale ... which would then date this into the 1960s rather than the 1950s.
And if that's the case then also maybe this isn't a dealer sticker as suspected.
Could it actually be a car club and 'BILLI' is an acronym with one of those 'L's standing for Lancia.
Just a thought...!