If you are using Amazon’s Fulfillment By Amazon service or any other third-party fulfillment for your inventory then you may have items that require two or more items together as a set, and separating them would cause the customer to have a bad experience with missing parts or a missing item. Just placing items that are supposed to sell as a set together does not mean that the third-party fulfillment center will know that. They could separate the items and when the set sells your customer would receive only part of the set they ordered.
What I do is put these neon orange (very noticeable) labels that say, “This Is A Set Do Not Separate” on these sets. For example, if it was two bottles of shampoo that sell as a bundle, I would poly bag them together and put a “This Is A Set Do Not Separate” label on the bag. These labels are very inexpensive and can save you the expense of a return or a negative feedback from items in a set getting separated.