A problem with this SBSTransit ad: it should read Relax, Mom; the next bus arrives in 8 minutes, because the boy is addressing his mother directly (otherwise it sounds like Mom is the object of the transitive verb relax, e.g. Relax your shoulders).
Similarly, when we greet or address others, we need a comma after the greeting:
Why don’t we use a comma in Dear Yasmin, then? Because dear is not a greeting — it is actually an adjective. Note that we can say My dear Yasmin, which is a typical noun phrase with the structure Determiner + Adjective + Head Noun.