My publishers are making my book available to amazon at a discount of 40%, and this has reduced my royalty payments.
As to other items, the only rule of which I am aware is that all items are to be priced lower than the amazon price. You can always verify this direct from amazon.
In general, you need to increase your unit price in order to maximize your profit. Also, you need to minimize your storage, postage, and packing costs.
Yet another problem is that the amazon delivery system is so reliable that a very large proportion of buyers still choose to buy from them, even when their price is slightly higher. But perhaps you could negotiate a lower price from your suppliers.