If your shipping has other restrictions other than international shipping limits, let your customers know.

For example, if you cannot deliver to certain states, or to P.O. boxes, make this clear.

Looking at the Michael's Shipping Policy again you'll notice that restrictions are listed for its UPS Next Day Air shipping service. Customers are informed that orders under this shipping method only ship on certain business days, that orders cannot be shipped to P.O. boxes under this method and that not all shipping methods are available for all items:

This information is crucial when it comes to managing customer expectations.