If the business you order from offers tracking, then your tracking information will show that your item will be returned to its origin.
Some vendors WILL reship, however there is no obligation to. Check with your vendor just in case. You may be required to send money for additional postage, which I understand. The pricing markup for some items is very low, so to expect them to LOSE money on a sale doesn't make a lot of sense.
If items are turned back, it does not guarantee that the items will be received by the vendor as well.
When I worked for Vapor4Life, our policy was that if your package was returned via Canada Customs, we would gladly reship on our dime once. If it is turned away a second time, you'd recieve a full refund.
The vendor in question ate the shipping fees too, so I always thought it was a classy move. You do get what you pay for in some cases.
That said, out of every package I have ever ordered from US suppliers, only once did I have something turned away. It's pretty low risk in my opinion.