I have problems concerning the tense. I find four tenses (present, past, present perfect and past perfect) in the following letter. I feel it is incorrect in letter writing.
Have any helper to resolve my problem?
(Grammar book: reported speech = past+past perfect, so the letter have past perfect tense, is it correct?)
Further to the captioned project, our company is a subcontractor under ABC company’s contract. Two nos. of 10tons of split type A/C unit installation are encompassed in our contract and the units for the contract have been purchased from your company.
Since the communication problem between the control panel and the a/c unit has been found during the installation period. The problem was still unresolved as a result of following the instructions on your product service manual a few times. Your technical staff has been invited to come on site to evaluate the problem but only was told that the solution had been published in the service manual and refused to provide further technical support to this problem.
In addition, a particular schedule for A/C units operation has been requested by client to suit the site condition. With limited technical knowledge of this product, we cannot perform this scheduled service. Therefore, we have contacted your company by phone for service support but your staff replied that the control unit service had not been included in the purchase agreement at the time of sale. We were also told by your staff that a phone consultation fee would have been charged but it was waived in a very rude manner.
We now write to express our dissatisfaction in the way your company handles the customer issues and concerns. We are extremely dismayed by your staff’s attitude during the whole telephone conversation. Your company has been requested several times to provide technical support on the products in order to assist our project handover to client. However, unprofessional service provided and poor working attitude of your staff push us to the utmost limits of endurance. We are surprised that as “XYZ” is one of the well-established companies in A/C industry in Hong Kong, your lack of professional support and responsibility is not what we expect from a business of your size.
The above matters have been arousing serious concerns for the client. Furthermore, we are considering taking your company out from our supplier list.
Four tenses problem 
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