It seems unlikely that you would send such an email to a customer, since it is about a customer or potential customer. (Actually you would be better to say client than customer. Customers are those to whom one sells goods, while clients are those to whom one sells services. However, this distinction is often ignored in business correspondence.) Perhaps you meant that you need to send an email to your head office. Anyway:
I need to send an (not one) email regarding one of our customers.
"I checked with the customer and they don’t need an official receipt for customs duty. The client wants to do long term business with us and therefore they have approached us. We need your support, as this is a valuable potential client. Please provide good rate and support us.
Thank you very much for your kind co-operation and we hope to receive a positive reply."