Sit with the client and document their needs. If what you wrote above is all you have, then you don't have enough to start writing code. Have them go through the steps manually one at a time, document them. Once you have it written down properly the coding will be a minor part of the project.
The "Documenting their needs" step is passed long back, the client is live now with NAV, it was a requirement raised by Sales Manager that if this automatic archiving is possible or not, since now he has to go and manually archive them one by one.
Just becasue the client is live, does not mean that any further changes should not be properly designed. All changes no matter how small and simple they may seem at first, can have big impacts on the system.
Without any information such as version, I don't exactlly know if this will help, but its a start. If you select Archive Document on the Sales Order Confirmation Report, if you select Post and Print, it will auto-archive the Sales Order. I know it wasn't your exact requirement, but its a start.
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The "Documenting their needs" step is passed long back, the client is live now with NAV, it was a requirement raised by Sales Manager that if this automatic archiving is possible or not, since now he has to go and manually archive them one by one.
Thanks