Since you have very little time to improve, I would directly come to the point.
Although your Essay follows the standard 5 paragraph template and includes a lot of transition words, still the flow of thoughts is not smooth. The language is too repetitive(for example count "the author should"), the sentence construction is lengthy, the event sequence is haphazard, and the essay is monotonous in overall.
The sequence should be:
Paragraph 1: Explain what is the argument (Context and conclusion). Then state the evidences. Just mention that the the line of reasoning is incorrect and there are questionable assumptions. You need not include assumptions here.
Paragraph 2: This must state the assumption, as the assumptions are perquisite to the argument.
Paragraph 3: Attack the line of reasoning, present counter evidence.
Paragraph 4: Suggest, how the argument could be improved
Paragraph 5: Sum up all of the above and state your conclusion.
Coming to your essay:
Argument states that deterioration of postal service can be reversed by increasing the price of postal stamps. Author assumes that with the increase in price of postal stamps will gain larger revenues to the postal department. Also author presumes that increase in price of postal stamps will reduce the volume of mail which, in turn, reduces the strain and contributes to improve the morale. Argument is flawed because of the blind assumptions author has taken on increase of postal stamps as a solution to reverse postal service deterioration, in addition author assumes price will help in development of revenues and eliminates strain.
This could be re-framed:
The argument claims that deterioration of postal service can be reversed by increasing the price of postal stamps. In support of the claim, the author states that the increased price of postal stamps will lead to large revenue and reduced mail volume, which, in turn, will help to eliminate the strain on existing system and improve employee morale. Stated this way, the argument reveals examples of leap-of-faith, poor-reasoning and fails to consider several key factors based on which the argument can be evaluated. The conclusion relies on questionable presumptions; hence, the argument is weak, unpersuasive and flawed.
The italic part can be memorized and used in almost all the arguments. The less you have to think, the more you can save time.
Practice other paragraphs in similar way and include variety, for example:
Instead of saying "the author should" - you can say "the argument could be improved/strengthened" , "the reasoning could be logical" , "the claim could be corroborated" etc.
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, follow how the sample essay has been framed using the template.
All the best for your test. Hope I have helped.
"Appreciation is a wonderful thing. It makes what is excellent in others belong to us as well."
Press Kudos, if I have helped.