Learn Hardcore Japanese: deeta wo hotondo tukawanakatta ue ni, tyoukaryoukin made aru. - I hardly used any data, and on top of that there is even an overage charge.

データ　を　ほとんど　使つかわなかった　うえ　に、　超過料金ちょうかりょうきん　まで　ある。deeta o hotondo tsukawanakatta ue ni, choukaryoukin made aru.

I hardly used any data, and on top of that there is even an overage charge.

Breakdown of deeta o hotondo tsukawanakatta ue ni, choukaryoukin made aru.

をo

direct object particle

Questions & Answers about deeta o hotondo tsukawanakatta ue ni, choukaryoukin made aru.

What does うえに mean here?

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.

Why is it 使わなかったうえに instead of 使わないうえに?

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.

What does ほとんど mean, and why is it used with a negative verb?