On Amazon is very easy to add your entire inventory with one button. Here you will see 2 options for you to upload products. The easiest way is by flat file upload.
Flat file upload
The flat file upload feature is a type of bulk upload. The flat file feature can be very useful for adding your entire inventory to Amazon in one go. It is a simple way to increase profits by using flat files to create and update your listings. The use of flat files can save you hours upon hours when it comes time to update your listings on Amazon. Using flat files also allows you to be more efficient in generating new product descriptions or titles and updating prices and shipping costs.
Merchant listing data
Inventory reports are snapshots of your listing information on Amazon. The reports show the items you have listed on Amazon at the time the report is run. With this, you can find old products that are stored in the document and use it to upload them again.