[Avodah] havdalah before megillah

[Eli Turkel]

This motzei shabbat for most of us is Purim.
Many shuls have a break where people go home after mincha for
seuda shlishit and then maariv is late to give kids time to dress up
and to bring a megillah to shul.
Thus one says "baruch hamavdil" and then does melacha including driving to
shul.

When does one say havdalah over the cup?
I haven't seen it discussed in the standard sources. On one hand it is
strange to say havdalah over the cup before maariv and megillah
(and drinking the wine)
also some people put off ending shabbat. On the other hand why
do all the work and drive before havdala when one can easily make
havdala.

-- 
Eli Turkel