I am not sure if the following sentences are ok when writing a business letter. if you find any problems, could you please correct them? Thanks in advance.

1.We are most anxious to ensure that [B]we are making our best efforts to offer the best satisfaction to our customers.[/B

]2. Due to the limited inventory of this product,we suggest you place an order immediately


3. Settlement of this account, unfortunately, have been already overdue by over 30days.

4.it is our common process to check trade reference before new account's open.