When /aux post bid is set on and you select a price on the bid tab, the starting bid is calculated wrong. The unit bid price is incorrectly divided by the stack size.

Unit bid price should be 18g 55s, not 6g 18s 33c:

It's not a bug, this is for undercutting the stack prices of auctions, not the unit prices, in order to appear higher up in the default ah sorting which can only sort by stack bid price.

Ok, my assumption for this feature is that I would be able to undercut both buyout and bid price at the same time so that my auction will be the highest in default sort. But don't understand how bid and buyout price can be completely different when the difference between lowest bid and buyout is couple of silver.

So if I post with these settings the buyout price then ends up being double of the currently listed lowest buyout (1g 24s 3c, the 2 top auctions are mine):

So selecting the lowest price on both panels with a stack of 2 will post an auction where the buyout price is 2x of the current lowest listed buyout price.

You're posting a stack of 2. The lowest has a stack bid price of 63, so to undercut the stack bid price you need a unit bid price of less than half of that for the bid.

The default ah does not sort by the buyout price, only by the bid. That's why the buyout price is undercutting the unit price, not the stack price, because only addons sort by buyout price and they will typically sort by unit buyout price because that's more useful, so you will still appear higher even if your stack buyout price is higher. As you can see, in the default AH, your auctions are appearing at the top when sorting, even though they have higher buyout prices. In e.g. aux they also appear at the top because they have the lowest unit buyout price.