deeta wo hotondo tukawanakatta ue ni, tyoukaryoukin made aru.

うえに means something like on top of that, in addition, or to make matters worse.

In this sentence, it connects two negative points:

• データをほとんど使わなかった = (I) hardly used any data
• 超過料金まである = there are even overage charges

So the whole structure is saying:

• Not only did I hardly use any data, but on top of that, there are even overage charges.

A key nuance of うえに is that it often sounds like the second point is being added as another reason something is good or bad. Here, it adds another complaint.

Because the speaker is talking about a completed fact in the past: did not use.

• 使わない = do not use / will not use
• 使わなかった = did not use

Since the sentence refers to what happened, the plain past negative form is natural before うえに.

So:

• データをほとんど使わなかったうえに…
  = On top of hardly using any data...

This shows that うえに can follow plain verb forms, including past forms.

ほとんど means almost, hardly, or nearly depending on whether the verb is positive or negative.

With a negative verb, it usually means hardly or almost not.

• ほとんど使わなかった = hardly used
• literally: almost didn’t use

This is a very common pattern in Japanese:

• ほとんどない = there’s hardly any
• ほとんど行かなかった = I hardly went
• ほとんど食べていない = I’ve hardly eaten

So in your sentence, データをほとんど使わなかった means I hardly used any data.

Here, まで means even.

So:

• 超過料金まである = there are even overage charges

This is not the まで meaning until.
Japanese まで has several uses, and this one expresses surprise, emphasis, or something extra beyond expectation.

Compare:

• 水まで飲んだ = I even drank water
• 彼まで来た = Even he came
• 超過料金まである = There are even overage fees

The nuance is: That’s bad enough, and it even goes as far as having extra charges.

ある here means there is/are or exist.

So 超過料金まである literally means:

• There are even overage charges

That sounds natural in Japanese. It focuses on the existence of such charges as part of the situation.

You could also imagine expressions with かかる in other contexts:

• 超過料金がかかる = an overage fee is charged / applies

But ある is perfectly natural when the speaker is basically saying, To make things worse, there are overage charges too.

Because Japanese often omits subjects and topics when they are understood from context.

Depending on the situation, this sentence could mean something like:

• I hardly used any data, and on top of that, there are even overage charges.
• We hardly used any data, and on top of that, there are even overage charges.
• This plan hardly got used for data, and on top of that, there are even overage charges.

English usually needs a subject, but Japanese often leaves it out if it is obvious.

Yes, データを使う is a normal expression.

• データ = data
• を = marks the direct object
• 使う = to use

So データを使う literally means to use data.

In contexts like phones and mobile plans, this often refers to using mobile data or data allowance. So the sentence is probably about a phone bill or plan.

超過料金 means overage charge, excess fee, or additional charge for exceeding a limit.

It is made from:

• 超過 = excess / exceeding a limit
• 料金 = fee / charge

So in a mobile-plan context, 超過料金 means the extra fee charged after going over your limit, or a fee associated with exceeding some allowance.

No. うえに can be used for both positive and negative things. It simply adds another point.

For example:

Positive:

• この店は安いうえに、おいしい。
  = This restaurant is cheap, and on top of that, delicious.

Negative:

• 彼は遅刻したうえに、謝らなかった。
  = He was late, and on top of that, he didn’t apologize.

In your sentence, it is negative because both parts are complaints.

Not with exactly the same meaning.

• ほとんど使わなかった = hardly used
• ～しか使わなかった = used only ~

しか～ない needs a specific limited amount or item before it. For example:

• 1GBしか使わなかった = I only used 1 GB
• 少ししか使わなかった = I only used a little

But ほとんど使わなかった is more natural when you just want to say hardly used any without naming a precise amount.

It breaks down like this:

• データを = data + object marker
• ほとんど = hardly / almost
• 使わなかった = did not use
• うえに = on top of that / in addition
• 超過料金まで = even overage charges
• ある = exist / there are

So the structure is:

[Clause 1] + うえに + [Clause 2]

More literally:

• I hardly used data, and on top of that, there are even overage charges.

Natural English might be:

• I hardly used any data, and on top of that, there are even overage charges.
• I barely used any data, and to make matters worse, there’s even an overage fee.

Yes, it does.

Without まで, the sentence would be:

• 超過料金がある = there are overage charges

With まで, it becomes more like:

• 超過料金まである = there are even overage charges

So まで adds a feeling of surprise, frustration, or disbelief. It helps show the speaker’s attitude: Not only this, but even that too?

That emotional nuance fits very well with うえに.