Bit of an interesting request regarding vendor bills Users like to enter the total (Header) and then the lines because Netsuite will let them know if there is an difference. However during an edit if you modify the lines on the vendor bill the Amount (Header) will automatically be updated. They would prefer it not to change and instead just give them an error when they go to save again. This seems like a base level feature from NS.

maybe a custom field for the anticipated total, and let NS do it's native behavior - the end user would see both their initial total and the cumulative NS calculated one for comparison?

Yes and you can use a WF action of Show Message to do a pop up if the amounts don't match

yea that sounds like a good idea.