We're shipping from Canada. Do we have to worry about sales and use tax?
Sure you do. If the shipment is being delivered in a US state, you'll have to worry about use tax if you have nexus in that state. Similarly, a US company will have to worry about Canadian and provincial taxes when it does business in Canada. The only issue is whether or not you have nexus in the US state you're shipping to.
If you don't have nexus, you have nothing to worry about. The buyer will have to self-assess their use tax. But if you do have nexus in the state, you'll have to register and collect taxes.
Sales Tax Guy