Want to add to this. We are right now being harassed by Amazon Seller Performance Team to comply with their ridiculous S-4 "Parity with your other sales channels" policy, which basically states that you have to keep the same prices on Amazon as on other websites.
So they can dominate you in shopping comparison engines.
Yet they take 15% commission.
This is BEYOND RIDICULOUS.
We are in the same boat as OP's client, we occasionally get a complaint or two which puts us over 2% (and in our industry it is BEYOND TOUGH, you tell other competitors there's only 2.5% complaints and they will laugh you out of the building. One product for instance is made by GE and they just pulled it off the market due to over 30% returns rate...). We are seriously looking to pull our products off Amazon, because I think we will be forced to.