No, they don't have to use Upstock – you can order from any and all your current suppliers. For suppliers not using Upstock, we send them your order in a structured email, including their exact product codes.
Yes, and this will simplify things a lot for you. Login once to do all your orders for all your stores, simply switch stores with a tap.
We offer paid upgrades for premium functionality like reporting and multi-site management, for a low monthly fee.