It's sent by using China Post Ordinary Small Packet Plus which is a cheap service of China Post. China Post doesn't provide tracking service for it after it moves for some stations in China. So, you can't get more update after May 13.
It should be on the way to Slovenia now. If you couldn't get it before the delivery deadline, better open a dispute in time to get refund.
My aunt has sent me a shipment from USA through USPS to Saudi Post. It’s been 2 months since she has sent the shipment to the shipping office and to this day there is no clue when or where the shipment has arrived.