Only for US use. The sticker is proof of payment of the entry fee:
FAR 24.22
(e)*Fee for arrival of a private vessel or private aircraft—(1)*Fee.*Except as provided in paragraph (e)(3) of this section, the master or other person in charge of a private vessel or private aircraft must, upon first arrival in any calendar year, proceed to CBP and tender the sum of $27.50, as adjusted in accordance with the terms of paragraph (k) of this section, to cover services provided in connection with all arrivals of that vessel or aircraft during that calendar year. Either a properly completed CBP Form 339V (Annual User Fee Decal Request—Vessels) or CBP Form 339A (Annual User Fee Decal Request—Aircraft), must accompany the payment. Upon payment of the annual fee, a decal will be issued to be permanently affixed by adhesive to the vessel or aircraft, in accordance with accompanying instructions, as evidence that the fee has been paid. Except in the case of private aircraft, and aircraft landing at user fee airports authorized under 19 U.S.C. 58b, all overtime charges provided for in this part remain payable notwithstanding payment of the fee specified in this paragraph.