Soo ...
I had a great idea for how to restructure the post tab (I hope you'll agree xD).
Here's how it works now:
The pricing model dropdown is gone. The behavior is now always like it used to be in the fixed price model in the sense that your price for the item will always be remembered and Aux will never automatically change the price.
The historical pricing model has been replaced simply by a button which sets the price to the historical price.
There's now a checkbox which lets you hide the starting price. If you check it the same as the buyout will always be used for the starting price.
And most importantly, undercutting now works like this: You can undercut a buyout price by clicking on a buyout price in the listing and you can undercut a bid price by clicking on a bid price in the listing. Left click will undercut the unit price (usually what you want) and right click will undercut the stack price (usually it would make more sense to simply reduce your stack size if too many auctions have a lower stack price despite a higher unit price)
There's no longer such a thing as "selecting a row" from the listing. As a replacement the prices in the listing are now colored, indicating three special cases. A price colored in red is undercutting your current unit price and has the same stack size, a price colored in orange is undercutting your current unit price but has a different stack size and a price colored in yellow has a higher unit price than yours but is nonetheless undercutting your stack price.
This color coding is available regardless of whether you use undercutting or not and is probably the most useful new feature. It will update whenever you change a price.
As always after large post tab changes double check your prices for a while after posting and please report any bugs you may find.