Hi. Can you please add a feature to multiple buying like in TSM? There I can enter any quantity I need. In your addon I can't set 39 at one time, I must click 39 times to achieve that.

Why not just shift click the item to my multiple?

Because it doesn't work.

Strange, I just tested it, and it seems to be working as expected.
What happens when you try buying it (I can see the dialog in your 2nd screenshot, so it looks like the dialog is showing up at least)

Nothing happens. I disabled all addons and no any modifier working. I can't multiple buy even Draconic Vial in default Blizzard's UI. But TSM can do this.

Sounds weird, since the default UI allows it.
Wthout being an expert on the extended UI, I think it would break things if it implements that, but maybe @TheKrowi knows? :)