How to minimize stt?

HI..to all i am new to trading ..today i just did sample trading to know how to calaulate brokerage,and other charges ..i woul like to explain this thing in more detail..so here we go........
i did trading on two shares namely..3I INFO and GMR (one trading on GMR one trading on 3I INFO) ..and after adding brokerage charges my loss was Rs.87 other charges are like this..

service tax Rs. 1.24
other charges Rs. 0.75
stamp duty Rs. 0.36
stt Rs. 2.00

So total profit was 87.00+1.24+0.75+0.36+2.00= Rs.91.35

Now day before yesterday my trading stats was like this...2 trading on GVK POWER 2 trading on IDBI..after adding brokerage charges my loss was Rs.11
and other charges are like this...

service tax Rs. 2.47
other charges Rs.1.43
stamp duty Rs. 0.68
stt Rs. 4

So net loss Rs. 19.58

Now what i want to ask by providing you all this statistics is that on loss of Rs.91.35 stt is Rs.2..but why it is Rs. 4 when net loss is Rs.19.58 how to minimize it? does it depends on number of trading like in first case i did 2 trading while in second case i did 4 trading ..please enlighten ..please help what should i try to do in order to minimize the stt ...????????