[mothered]

It's going to be a partial because I know that works best, so I have the 3 items that I'm really wanting in my basket here:
So what else do I need to add to my basket? Do I just add 3 other sets of shirts or socks? Would shirts work best?

What's the weight of the Item? The success rate of the partial/missing Item method, Is always based on the weight.

In terms of what you wish to add, It's completely Irrelevant. Whether you have 3 Items or 20, Is Immaterial- they'll simply deduct the weight of the Item you're SEing from the total consignment, and calculate the difference (If any). That will determine If the Item was dispatched with the rest of the goods.