Complex sentence: However, this was in turn restricted by tariffs that one had to pay in order to put up posters in authorised spaces, thus posters had to be memorable even to someone strolling past and therefore maybe only glimpsing it once.
Split into simpler sentences: However, this was in turn restricted by tariffs that one had to pay in order to put up posters in authorised spaces. Posters thus needed to be memorable even to someone strolling past and therefore maybe only glimpsing it once.