Due to / for bookkeeping reasons we are (unfortunately) unable to fax a copy (of ...) [through] to you.

We regret being unable to fax a copy (of ...) [through] to you. This is due to / for bookkeeping reasons.

Ich würde Folgendes vorschlagen:

Due to / for bookkeeping reasons we are (unfortunately) unable to fax a copy (of ...) [through] to you.
-> evtl. photocopy (Fotokopie) oder carbon copy (Durchschrift)

Oder (etwas freundlicher): We regret being unable to fax a copy (of ...) [through] to you. This is due to / for bookkeeping reasons.

Ich denke, accounting ist auch ok.

I like Florian's suggestions, but "Bookkeeping reasons" does not sound normal.

Is this
a) a vague and feeble excuse for not sending something the customer has a perfectly good right to receive, or
b) an explanation of why something has been recorded in a particular way, that makes it impossible to do what's been requested.

If (a): "administrative reasons" (very vague)
If (b): "accounting reasons"

Thanks alot for your help