Hello,
Could you take a look at this complaint letter, and add your corrections, regarding my own ones as well.

Dear Sir/Madame

I am writing to you in order to point out that there were (I think – have been) some impermissible (inexcusable) mistakes in your work.
We ordered 10 laptops in (I think – from) your company, but to our huge disappointment we have not got (I think – we did not get them) in time. This situation caused a lot of problems in the working process in our company and a lot of costs (which resulted in additional expenses), so we would be grateful if you delivered the orders in terms (I think – on time). We are extremely dissatisfied with such a poor service that is why in future we will need a guarantee with delivery dates.
I am going to inform you that if there is a repetition of late deliveries we will have to look for another supplier.
Thank you for your attention.

Yours sincerely,
Kate Soboleva
Manager

Thank you in advance