Johnny,
I use Copy/Paste heavily for Sales or Purchase Lines. You can take your Sales Line page, insert 1 line of data that matches what you require, then click, on the Line action bar, Page/Edit in Excel. Now in Excel make sure the columns are in the Order your page appears.
Sales Order:

Now Excel:

Now go back to Sales Order, with your Mouse
and Paste the rows highlighted from Excel, without header

Hope this helps. One last thing, the Copy Document function will help going forward to copy previously posted invoices or archived sales orders to duplicate. If you are trying to correct a previously posted invoice, yo can use the Correct function from the Posted Sales Invoice too.
Thanks,
Steve