Wir bitten um Rechnungskorrektur, zumal der Arbeitspreis doppelt ist.

In German, bitten can be used in two common patterns:

• jemanden bitten, etwas zu tun = to ask someone to do something
  
  • Wir bitten Sie, die Rechnung zu korrigieren.
• um + accusative = to ask for something
  
  • Wir bitten um Rechnungskorrektur.

In your sentence, the speaker is asking for a correction, so um is required.

German often leaves out the article in formal, business, or administrative language when referring to something in a general or official way.

So um Rechnungskorrektur sounds concise and businesslike.
You could also say:

• um eine Rechnungskorrektur = asking for a correction, less compressed
• um die Rechnungskorrektur = referring to a specific correction already known in context

Without an article, it sounds like standard office or email German.

German very often combines nouns into a single compound noun.

Here:

• Rechnung = invoice, bill
• Korrektur = correction

So Rechnungskorrektur literally means invoice correction or correction of the invoice.

This is completely normal in German. English usually uses separate words, but German prefers compounds.

Zumal is a conjunction meaning something like:

• especially since
• particularly because
• all the more so because

It introduces an additional reason that strengthens the main statement.

So the structure is:

• Wir bitten um Rechnungskorrektur = the main request
• zudem / especially since ... = an extra reason for that request

Compared with weil, zumal often sounds a bit more formal and emphasizes that the following reason is especially relevant.

Because zumal introduces a subordinate clause.

In German subordinate clauses, the finite verb usually goes to the end. So:

• Der Arbeitspreis ist doppelt.
  This is a main clause.
• ..., zumal der Arbeitspreis doppelt ist.
  This is a subordinate clause, so ist moves to the end.

This is the normal word order after conjunctions like weil, dass, obwohl, zumal.

Because zumal introduces a subordinate clause, and German normally separates subordinate clauses from the main clause with a comma.

So:

• Wir bitten um Rechnungskorrektur, zumal der Arbeitspreis doppelt ist.

That comma is required in standard German punctuation.

It is nominative, because it is the subject of the subordinate clause:

• der Arbeitspreis = the labor charge / work price
• ist = is
• doppelt = double / twice as high

So der Arbeitspreis is the thing doing the being in ist doppelt.

Arbeitspreis is a somewhat technical or business-related word. Depending on context, it can mean:

• the labor charge
• the charge for work performed
• a unit rate for work or usage

In invoice contexts, it often refers to the amount charged for labor or for the service component, rather than materials.

So a learner should understand it as a specific pricing term, not a very everyday word.

Here doppelt means double or twice as high.

In der Arbeitspreis doppelt ist, it functions as a predicate expression after sein. German often uses adjectives or adjective-like words this way:

• Der Preis ist hoch.
• Der Preis ist falsch.
• Der Preis ist doppelt.

So doppelt is describing the state of the price: it is double.

In natural English, you might often translate it more freely as has been charged twice or is twice as much depending on context.

It is understandable, but in many real invoice complaints, German speakers might choose something more explicit, for example:

• ..., zumal der Arbeitspreis doppelt berechnet wurde.
  = especially since the labor charge was charged twice
• ..., zumal der Arbeitspreis doppelt angesetzt ist.
  = especially since the labor charge has been entered at double rate

Your original sentence is not wrong, but it is a bit compressed. The exact best wording depends on whether the price itself is too high, or whether it was literally billed twice.

Yes. Wir bitten ... is a formal and polite business style.

In letters and emails, German often uses wir when speaking on behalf of a company, household, or team. It sounds professional and less personal than ich bitte.

It is also less direct than an imperative such as:

• Korrigieren Sie bitte die Rechnung.

So Wir bitten um Rechnungskorrektur is a polite formal request, suitable for customer service or invoice-related correspondence.

Yes. German business language often prefers noun-heavy phrasing, but a verbal version is also possible.

For example:

• Wir bitten darum, die Rechnung zu korrigieren.
• Wir bitten Sie, die Rechnung zu korrigieren.

These are a bit less condensed and may sound more straightforward.
The original sentence uses the noun Rechnungskorrektur, which gives it a more formal, administrative tone.