If not did someone go through the LOA process? Was it a smooth one or no chance?
Yes, you absolutely only need one barcode, but they all must be packaged together.
You can bundle multiple brands into one gift set on Amazon if the products are complementary and you have the required brand approvals.
The LOA process can be smooth with the proper documentation, but it can be challenging if the brands are restrictive. Some sellers have managed to navigate it successfully.
In most cases you cannot bundle multiple brands into one gift set unless you have explicit written authorization from each brand owner, because Amazon treats this as creating a new bundled product and requires brand approval for every brand included.
Without Letters of Authorization, these listings are very likely to be taken down sooner or later, especially if any brand is enrolled in Brand Registry or files a complaint.
Sellers who have gone through the LOA process report mixed results: it can be smooth if you are working directly with cooperative brand owners who understand Amazon and are willing to grant written permission specifically allowing bundling and resale on Amazon Seller Central, but for well-known or tightly controlled brands it is often very difficult or effectively impossible. As a result, many sellers either bundle only unbranded items, bundle products from a single brand, or create their own private-label brand to avoid enforcement issues entirely.