How do I manually synchronize products?

Product Synchronization referred to in this article is for products already that already exist in your store. To push new listings to your store, please refer to the 'Product Catalog' section. 

Spark Shipping will automatically synchronize with the products in your shopping cart every 24 hours.

You can see when the last time a products sync had ran on your dashboard. You can also manually force a products refresh on the same screen: