Non delivery of Collection slips

Danhof post office, does not deliver my slips. They will deliver my slip when it says final slip. I ordered a few items from Wish.com last year and have not received my items, which has been 7 months already. Last week I received a collection slip in my poxbox. It was stamped on the 3 January 2018 at Danhof postoffice, but it only arrived in my postbox on the 26th January??? And it was a final collection slip. When I went to the postoffice to collect, they said that it was apparently sent back? I have lost 4 items that I have ordered from Wish.com because they don't send out the collection slips. Really bad service!!