Amazon seller shipping credits are flat for books, and are not calculated by weight or the location you will be shipping to. You will be credited the same amount for both a two ounce leaflet and a 15 pound textbook.
Within the United States, there are a few Amazon shipping methods.
For items weighing 7 ounces or less, you will save the most shipping via First Class Mail versus shipping that same item by Media Mail. For items weighing more than 7 ounces, shipping costs less by Media Mail versus First Class Mail.
You can ship via Media Mail for the “Domestic Standard” option, which is not at all unreasonable. With the $3.99 credited to you, you will be able to ship up to a 5 pound book ($3.94 via Media Mail) without losing any money.
When shipping “Domestic Expedited”, you will be shipping via Priority Mail. You will be credited $6.99, but to ship that same 5 pound book via Priority Mail to California if you are located in Maryland, it will cost you $17.65 ($10.70 if you can fit it into a Medium Flat Rate Box). Priority Mail costs are variable according to your shipping zone and the zone you are shipping it to. Amazon credits you $6.99, so you will be losing $10.66 to ship it.
If you choose to ship internationally, sometimes the shipping will cost a significant amount more than the Amazon international shipping credit you will recieve. If you are shipping via USPS, “Standard International” translates into USPS First Class International, and “Expedited International” translates into Express Mail International. Remember, the shipping credit will not be calculated by where you are shipping to or the weight of the book.
Here, I will be using Australia as an example for the shipping prices because when shipping from the USA the Amazon shipping costs to Australia are highest. Shipping is a bit lower to Europe, but not too much lower.
If someone from Australia orders a book from you that weighs 4 pounds and chooses the “Standard International” option, it will cost you approximately $31.40 to ship it to them via “First Class International”. Amazon credits you $12.49 in shipping, so you will be losing $18.91 to ship it.
If someone from Australia orders a book that weighs over 4 pounds, and chooses the “Standard International” option, the shipping rate to Australia will be $46.90 ($43.45 if you can fit the item in a Medium Flat Rate Box). Items weighing over 4 pounds cannot be shipped via “First Class Mail International”. For items weighing over 4 pounds, it is necessary to send them via “Priority Mail International”, even if the buyer chooses the “Standard International” option. Amazon credits you $12.49 in shipping, so you will be losing $34.41 to ship it.
If someone from Australia orders a book from you that weighs 5 pounds and chooses the “Expedited International” option, you will be shipping via USPS Express Mail International. Amazon will credit you $35.98, and the shipping cost via USPS Express Mail International will be $55.75, meaning you will lose $19.77.
You can ship up to a 1 pound 4 oz book to Australia with the amount credited by the “Standard International” option without losing any money. With the “Expedited International” option, you can ship up to a 15 ounce book to Australia without losing any money.
And remember, as it states on Amazon: “Shipment is required even if the shipping amount credited to your account does not cover all shipping costs.”