Circle.us is described as 'integrates with multiple selling channels and shipping carriers to streamline the order management, shipping, and fulfillment for online retailers. Manage and ship all your orders across multiple channels in a beautifully simple place'. There are more than 10 alternatives to Circle.us, not only websites but also apps for a variety of platforms, including Windows, iPhone, Android and Mac. The best alternative is Veeqo. It's not free, so if you're looking for a free alternative, you could try EMERGE App or Tana Inventory Management. Other great sites and apps similar to Circle.us are Shiptheory (Paid), Primaseller (Paid), ShipHawk (Paid) and ShipStation (Paid).
Primaseller is cloud based POS Software which Integrates with multiple online channels of sale synchronizing inventory and orders from a single platform. It also integrates with Shippers, QuickBooks online and Multiple Payment Methods.