SHEIN not taking accountability for paid missing item from package delivered. Unsatisfied SHEIN customer.

I have made an online purchase from Shein and an item which I paid for was not sent in my package, meaning my package arrived with a missing item.I escalated this to their relevant department and so sad for the pass week they have been prolonging this case and insisting that I must do a follow up with THEIR logistics provider. How is this my responsibility has THEIR customer? I'm very disappointed has a long term loyal Shein customer with their assistance. Irrelevant to the cost of the item that's missing, this is scary for customers who buy expensive Items which could also go missing and this will be the sad response from Shein. Furthermore I purchased toddlers shoes and clothes which was advertised for 2/3 year old ,but arrived fitting a six month old baby. Unfortunately for me again no exchange or refund on those items. What should I do with does items? Very disappointed and will certainly not be supporting Shein in the future has they have not given me no joy with their sad assistantance.