I have nothing to declare but my genius

Wilde did visit the USA and this quotation is certainly in keeping with his style. There's no evidence to prove conclusively that he said it though and it doesn't appear in print until 1914. His US tour was very successful and Wilde became quite famous there, so it is also possible that the 'quotation' was made up for him.

There are many quotations attributed to Wilde, for example, "I can resist everything except temptation" and "We are all in the gutter, but some of us are looking at the stars", and many of them are bogus. Wilde was clearly highly adept at coining repeatable phrases and those two did in fact come from his pen - from The Importance of Being Earnest, 1895, and from Lady Windermere's Fan, 1892, respectively. Here are a few others that can't be questioned, as they also come from his published works:

"Life is much too important a thing ever to talk seriously about it."

"To love oneself is the beginning of a life-long romance."

"Women are meant to be loved, not to be understood."

Statue to Oscar Wilde in Merrion Square, Dublin (nicknamed 'the Quare in the Square').

See also - 'the floozie in the jacuzzi'.