SHEIN offers FREE Standard Shipping on all orders. Standard Shipping time is 6-8 Business days (to US). You can opt to pay for Express Shipping which will arrive in 2-4 Business days. Express Shipping is offered for FREE if the order value is over $99, otherwise costs $12.90.
Please note there is also the "Processing Time" to take into account when communicating with your customers. Processing times are typically between 3-4 days but can be up to 7 days.
Processing time + Delivery time = Receiving time
You can learn more about their process here.