SYI form is pretty fussy.
Many times when changing the format in "Sell Similar", it would not change offer proper parameters (e.g. it would be showing only Buy Now when switched to auction) when switching for the first time, you need to switch back to original format, wait a little and then switch to new format.
The Lot Size box requires a few seconds wait when you switch into it, otherwise it will switch your value back to what it was. Actually, my most recent neg was given because we shipped 50pcs instead of 200 - I created a lot of 200 out of lot of 50, but SYI form switched it back to 50 so invoice would indicate 50pcs to ship. When I realized, I sent remaining 150pcs to all buyers who purchased from that listing, but this guy was not happy and demanding some weird $30 charge from me, which I refused of course.
Anyway - be cautious with SIY form. I use Chrome, it may behave differently with other browser.