Record ID

I have a 'Customers' worksheet that has assigned ID numbers for each customer in column 'A'. These ID numbers are used on the Invoice worksheets to fill in the customer information. My question is once these are assigned to that customer can they remain with them even after sorting the records? I have had issues with duplicate entries with customers is why I am asking.

If that is possible then could the 'Last NM' column be automatically sorted from A to Z when adding new records (customers)?

This seems to be a tricky issue for me so I appreciate the step in the right direction.

Any suggestions for my format is appreciated also.