1lvl cost ~3.8pln
10lvl cost ~42pln - description says pay for 9 get 1 extra, i have tested and got 10lvls but my math tells me that I have payed for 11lvls

Haven't tested 100lvl option but i assume that there is no logic as in 10lvl option.
Fix it since its better to buy 100x1lvl than 1x100lvl.
10lvl - pay for 10 get 11 for 38pln
100lvl - pay for 100 get 110 380pln
That would be more logic...