OK, found it in the variations tab.
There are manymany things that confuse me.
At one place I have to add the price including VAT, and the other without. I added Purchase Price without VAT and then it subtracts the VAT. There should be some note for this, somewhere.
Also, when using PO - when using "Add to stock" button, there is no hint that only the selected product is added to the stock. I discovered it later. And I didn't even understand, that if it's pink, then it's selected. There should be somewhere (near the "Add to stock" button) text like "1 of 30 selected" or something like that. I hope you can make these changes. Thank you!