Set Product SKUs

Basically, we need the ability to set individual SKUs for products, rather than have them be automatically generated by PayKickStart.

PayKickStart already integrates with ShipStation, but from there, many fulfillment partners work by looking for particular SKUs in ShipStation.

For example, orders will only move from ShipStation to Amazon FBA if the SKU matches.

If we could set the SKU on a product-by-product basis and then have the existing integration from PayKickStart to ShipStation send over the SKU, that would solve all of the current issues with the integration.