Mandarake is a chain of physical stores in Japan and they sell mostly second-hand items online from each of the stores. Their online store will tell you what store you’re buying from, so make sure you buy from the same one, or you will pay separate shipping for your items.
On each item, there will be the store that it is located in underneath. Try and find the same item in each store to avoid paying separate shipping for each item. For this example, I would get the first two items from the Fukuoka store and try and find the third in the same store, as the SAHRA store is in a different location.
If you’re unsure, add the items to your cart anyway, as it separates the items out based on where it will be shipping from. You can then make a decision on what items you want to buy. Remember, shipping is expensive, so you’re better off waiting until there’s a few items you want rather than buying just one or two, considering how cheap they can be.
I only saw one doujinshi I wanted to complete a set and ended up getting a few more items I’d had on my wishlist for a while.