Selling on Amazon FBA (Fulfillment by Amazon) is convenient, but fees can eat 30-40% of your revenue. Beyond the standard "Referral Fee" (usually 15%), you must account for "Fulfillment Fees" (based on weight and size) and "Storage Fees" (which triple during Q4/Christmas).
Your "Cost of Goods Sold" (COGS) isn't just what you paid the manufacturer. To calculate true profit, your COGS must include the shipping cost from the factory to Amazon's warehouse, customs duties, and packaging materials. A miscalculation here can mean you are selling at a loss without realizing it.