I have an ongoing issue with Multi Buy Orders. I did report this on the techincal issues part of the forum a few months ago.

For example a customer buys 5 items across different listings. It correctly calculates the 10% discount the customer should get but splits the items across 2 orders.

One of the reasons for offering a 10% discount is to pass the postage savings I get onto the customer. However as it splits it in 2 orders we sometimes ship as 2 seperate orders as it is difficult to identify in Royal Mail click and drop which orders need to be merged.

Ideally it should place the 5 items under the same order number.