So the manufacturer was just re-selling stuff from Amazon?
These manufacturer fulfilled network (MFN) orders cost the manufacturer a premium in shipping fees over FBA orders (orders sold/fulfilled through Amazon), but don't carry the referral fee (8-40%).
Given this, it is almost always cheaper for the manufacturer to sell off Amazon and ship MFN... if you ignore customer acquisition cost.
Referral fees for the vast majority of items is 8% to 15%, which in and of itself is a massive range. I believe this is mostly due to the typical margin on each item. Perhaps the typical cost of customer acquisition also plays a role.
The minimum is 6% for personal computers, which historically have super slim margins anyhow, and 45% for Amazon Device accessories, probably because Amazon wants to maintain a major advantage over third party accessories (and possibly because they dominate the market for Amazon device accessories, so they're able to charge a major premium)