A Formal Email

To wWhom iIt mMay cConcern,

For a formal email, these are always capitalized.

I am writing to express to you my dissatisfaction withover the latest change in theto your delivery policy.

First, I consider that this is a terrible and selfish decision.

This is very strong wording which we would normally never use. We'd write something more like this: "First, I consider this to be very inefficient and expensive."

To make matters worse, you have informed us about it only two days before the announcementimplementation, which meants some customers have not been able to realize ofprobably missed it.

In consequence, I would appreciate it if you would provide us the economic reasons ofor this decision in detail.

Additionally, I would like to present some ideas about how to approach this problemopose different options.

I think you shouldrequest that you provide us an alternative, as we are facing the rising fuel pricesprice of fuel as well.

Therefore, to resolve this situation, we would like a partial refund for those who have bought withmade purchases at these new prices.

I am writing to you, because I would like to express to you my dissatisfaction with the latest change in theyour delivery policy.

Stylistic changes. If this was a casual email, this wouldn't be as important, but you want to be as proper as possible.
The beginning isn't a necessary change - but in a formal email, it's good to use less direct language. You want to get your point across without sounding rude or snide. (The way you had wrote it wasn't bad, but this would be an improvement.)
I would change "the delivery policy" to "your delivery policy" as it sounds cleaner.

First, I consider that this isis to be a terrible and selfish decision.

Sounds more clear and clean. "Consider that this" just sounds a bit clunky.

To make matters worse, you have informed us only two days before the announcement, which meant that some customers have not beenweren't able to realize of itthe change in time.

I got rid of "have" to make the sentence more concise.
"Of" does not belong with "realize".
I added "that" before some customers to make it sound a little better.
"Realize the change in time" sounds much clearer and reaffirms the point you are trying to make.

In consequenceSo, I would appreciate it if you wcould provide us the economic reasons of this decision in detail.

I don't think "in consequence" works here.
Added "it" for grammar flow correction.
I changed "would" to "could" here, I think it sounds a bit more formal. This one is more personal and stylistic.

Therefore, to solve this situation, weI would like a partial refund for those who have bought (some of this company's goods or services) with these new prices.

This is missing something, but I can't tell because the rest of the email provides little context.
Changed "we" to "I" to be consistent with prior pronoun usage.

I would like to discuss this matter further with you as soon as possible before I canceling my premium account.

Personal stylistic choice.

To whom it may concern,.

I am writing to you to express to you my dissatisfaction with the latest change in theyour delivery policy.

Firstly, I consider that this isis to be a terrible and selfish decision.

To make matters worse, you have informed us two days before the announcement, which meants some customers havemay not been able to realize of ithave realised what has happened.

Not entirely sure what you are trying to say here so I may have phrased it wrong.

In cConsequencetly, I would appreciate it if you would provide us with the economic reasons ofor this decision in detail.

Additionally, I would like to present some ideas about how to approachsolve this problem.

I think you should provide us with an alternative, as we are facing the rising fuel prices as well.