Treasure: I do not know how to find ids for individual games/shelf entries indeed, so I'd appreciate it if you could tell me how I'll find that information. I could try and guess the lookup process, but I can end up with the wrong ids by improvising, so I'd rather follow the correct method...
No problem, I would have just typed it out but I was on my phone and wanted to make sure I told you to do the right thing.
If you right click on the game in your library (probably on the image is easiest) and click inspect it will bring up the html code. You need to look for something like this (it should be towards the top of the window that has opened or you might need to scroll up a little:
<div class="product-row js-details-pointer dimming-chooser__element product-row--has-action ng-scope" draggable="false"
gog-account-product="1937342852" gog-product-dropdown-pointer="1937342852" ng-class="{'is-hidden': product.isHidden, 'is-updated': product.updates}" ng-click="chooser.setChosenID(1937342852, $event)" style="box-shadow: rgba(0, 0, 0, 0.74902) 5px -5px 5px 0px;">
And the Id is the gog-account-product section (bolded) which in this case is 1937342852 (for Halcyon 6).
If you can get me those numbers for both of the Cossacks games I can fix them :)