English Bus/Financial Business/Commerce (general)

Please help to rephrase the sentence marked with asterisks to get rid of the repetition of case. Thank you!

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Dear Mr. Smith,

Further to our letters of December, 9 2007 and December, 16 2007 we are writing concerning
the outstanding invoice No. 1234567 for 1000 USD (see attached copy), which we expected
to be cleared before December, 2 2007.

So far we have not received neither the remittance nor an adequate explanation of the delay
from your side.

We feel that you have been given sufficient time to clear this balance and now we insist on
payment within the next two weeks that is before January, 12 2008.

***Otherwise, we will be forced to forward this case to the collection agency, with which we
co-operate on such cases.***

Sincerely Yours,