harrasment at the hands of sales tax officials

Since starting my business of air conditioning (say for last 15 years), every year I had been harassed by West Bengal sales tax assessing officers, who while assessing my tax, demanded money , and on refusing, willfully misinterpreted the acts and Laws, and demanded exorbitant tax.
one example is, we do undertake yearly service contracts, after one years free warranty service,( the service charges are a service tax subject,) However , every year for last 15 years, the sales tax officer assessed these charges as sales and taxed it accordingly , and after going for appeal ,the assistant commissioner accepted these are not taxable under sales tax laws, but I had to undergo mental agony, waste my time and money for the entire process.


Then every year I had to go for appeal, wasting my time and money, to undo the wrong assessment.
This time too on 3rd of january, I have received assessment order with the similar assessment, because I had refused to give hush money to the officer concerned.and I will again have to appeal and waste time.
Another example is misuse of works contract Law:
After selling air conditioners, some times we have to get it installed, we are paying full VAT tax on air conditioners sold, as per the works contract law, when supply and fitting part can't be separately assigned, hypothetically, sale tax officer has to take 65% of the value as value of ac, 20 % as value of accessories for fitting and 15 % as labour, and taxing them accordingly, however in my case , after refusing to pay hush money,they have judged installation charges separately as full sales, I had to fight and explain to AC while going for appeal, that if you apply the works contract properly, you will have to add the sales and fitting charges together and then tax 65 % of the addition as sales, then government will have to refund .as we have already paid fully on about 90 % of the addition.