Amazon recently added a new field to its FBA Fee Calculator asking how much inventory you will store on average (image below). This allows Amazon to give you a more accurate storage estimate by including short and long term storage fees.
If you plan on purchasing less than 90 days worth of inventory with decent profit margins, you can usually just leave this field blank because your storage fees will be minimal (just a few pennies per unit).
If you are planning on purchasing long term inventory, your storage fees will be much more important and you should then fill in the "Average Inventory Units Stored" field with the number of units you plan to store at FBA. If you do put a number in the Average Inventory Units Stored field, you will also want to enter "Estimated monthly units sold with Seller Fulfillment" (image below). Estimated monthly units sold with Seller Fulfillment are the number of units you expect to sell every month based on your analysis. With this info, Amazon will give you a better estimate of your profit based on storage fees.
If you plan on purchasing less than 90 days worth of inventory with decent profit margins, you can usually just leave this field blank because your storage fees will be minimal (just a few pennies per unit).