So you probably saw that the Obamas weren't invited to the royal wedding (I didn't go look this up, I would like to note, I couldn't avoid it...although I will confess that I will go check out wedding photographs after it is over although I will NOT be staying up to watch it). As you can see from this piece, American presidents do not normally attend royal weddings, even when invited, so this is definitely not a "snub."
Now Nancy Reagan did attend the wedding of Prince Charles and Lady Diana in 1981. What did the Reagan's give to William's parents?
Expressing the good wishes of the American people, President and Mrs. Reagan are giving Prince Charles and Lady Diana Spencer a wedding present befitting a future King and Queen. It is a magnificently engraved Steuben glass bowl. In addition, the Reagans are giving a handmade porcelain centerpiece by Boehm.
President Truman also gave Steuben glass to the then Princess Elizabeth on her wedding day:
After World War II, Steuben designer Walter Teague turned to the drawings of John J. Audubon to create a series of 10-inch plates, each of which featured an engraving of a different bird on its base. In 1947, President and Mrs. Truman presented a set of the Steuben Audubon plates to Princess Elizabeth as a wedding present.