This is an odd topic! Where did you come across it? It's interesting, though, and I really like your arguments against it. Your essay is clearly laid out and ideas are balanced and well developed.

Introduction

I love your introduction:

Keeping shops open the entire week and all day and night seems convenient for customers at first glance. Unfortunately, in my view there are some side-effects for most customers, shop staff, and the businesses that make long hours not sustainable in most store locations.

You give one side (the idea that shopping is convenient for customers). You use ‘at first glance’ perfectly. Have a look at the official IELTS descriptors for Task 2 Writing here, under Lexical Resource, and look at the phrases ‘skillfully uses…’ or ‘sophisticated control.’

There’s a tiny bit of repetition that would have been nice to avoid, but it’s not easy to rewrite!

Readers, notice how the thesis sentence doesn’t say “In this essay” or “This essay will.”  However, it’s an excellent example of a thesis sentence. There are three parts (customers, staff and shopkeepers), and each of them gets a paragraph in the same order in the essay. It's very clear what the essay is going to do and what we are going to read.

Word Choice/Word Form/Usage

• Both is known to be neither healthy nor family-friendly. → Both are known…
• it can be challenging to make the increased costs of longer opening hours back. →
  It can be challenging to recoup/recover/ get a return on /justify the increased costs of longer opening hours.
   
• Therefore, either prices have to be risen… → Therefore, either prices have to be increased/raised…
• wages for store stuff is reduced → wages for store stuff are reduced

Either: You shouldn’t really use ‘either’ with three items (it’s usually for a choice between two things):

• either profits drop, prices rise or wages for store stuff isreduced. → (let’s just leave it out!:
  profits drop, prices rise or wages for store stuff are reduced

Word Choice

You wrote:

Overall, in my view the disadvantages of longer store hours clearly dominate for all parties involved.

I know you are trying to avoid the overused ‘outweigh.’ Maybe this is one solution, and it avoids the passive:

Overall, in my view the disadvantages of longer store hours for everyone are clear.  Everyone pays more in the end.

• not sustainable → unstainable. You have a choice, but unsustainable might be better here. 

Parallel

The later will make the work more stressful and store stuff has less time to spend advising customers

Katiss, I’m surprised! You know I hate when people use ‘the latter!”  The second part of the sentence needs to be parallel in tense:

Being overworked will make the work more stressful and staff will have less time to spend advising customers.

Overall, another great essay. I think, Katiss, the reason that you are not getting the band you want is that examiners are looking at your essay negatively rather than positively: they start with a very high estimation, based on your fluency, your skillful use of less common vocabulary and the very high number of error-free sentences, but then they drop down when they see a simple agreement error or a less successful vocab choice. With weaker candidates, they may try to find something good: in your case they are trying to find something negative. Not very comforting, I know.

I hope the next time you get a more sympathetic examiner!

Keeping shops open the entire week and all day and nigh seems convenient for customers at first glance. Unfortunately, in my view there are some side-effects for most customers, shop staff and the businesses that make long hours not sustainable in most store locations.

From a customers perspective longer shop hours potentially come with higher prices and less staff present in the store at any time to compensate for increased costs. Most likely only a few customers would actually shop in the middle of the night regularly. Generally speaking, the convenience of longer store hours comes with higher prices and less service for all consumers.

For store staff longer opening hours may lead to longer working hours and working in shifts. Both is known to be neither healthy nor family-friendly. To offset the higher costs of longer opening hours shopkeepers are likely to cut the pay off their staff, employ less qualified staff or make less staff do the same work. The later will make the work more stressful and store stuff has less time to spend advising customers. As a result, working conditions are likely to deteriorate with longer store hours.

For shopkeepers it can be challenging to make the increased costs of longer opening hours back. Unless
a store is located in a location with lots of passer-by during late hours sales are unlikely to increase significantly through prolonged hours. Therefore, either prices have to be risen, which is only possible if completion permits this step or staff costs have to be reduced. Longer store hours increase the costs of running a store and can affect profits.

To sum up, the higher costs of longer opening hours will have to be offset in some way. With most stores unable to make to make them back through increased sales either profits drop, prices rise or wages for store stuff is reduced. Overall, in my view the disadvantages of longer store hours clearly dominate for all parties involved.