As has been mentioned to you before there is not such thing as FDA approval of laser sights as such. Compliance with all FDA rules and regulations is not approval just compliance and has only to do with FDA and the manufacturer--nothing to do with you and US Customs.
Really you need to contact the FDA and get real answers to your questions about a specific product and specific manufacturer.
Here is part of a response from the FDA to one guy asking about laser sights--maybe it will give you some insight---the entire post can be found here:
IR Laser Law - Response from the FDA - Page 3 - AR15.COM
"[Laser sights are subject to the provisions of 21 CFR 1040.1 1(b), Surveying, leveling and alignment laser products. This section limits this type of laser product to Class IIIa and a maximum output of 5mW. If they exceed the 5 mw limit they are required to comply with all applicable Class IIIb requirements which include an emission indicator, key switch, remote interlock connector, specific labeling and other features. Laser sights in the visible spectrum are limited to IIIa. If the manufacturer wants to power them above IIIa, they need to get a variance. IR lasers as surveying, leveling and alignment lasers are limited to Class I. To be powered above Class I as a laser sight, the manufacturer needs a variance.
Class IIIb aiming devices cannot be legally sold in the U.S. without a variance from the applicable requirements of 21 CFR1040.10 and 1040.11 having been issued by the Center for Devices and Radiological Health for such devices. It is our policy to restrict the sale of Class IIIb aiming devices only to the military, government and law enforcement agencies by purchase order for the procuring company.
An application for a variance may be made following the guidance found in 21 CFR1010.4 (b). A complete laser product report using the FDA's "Guide for Preparing Product Reports for Lasers..." must also be submitted.
CDRH issues to certain manufacturers, a variance from certain requirements of 21 CFR 1040.10 and 1040.11. The basis for approval of the variances is found in 21 CFR 1010.4(a). These variances apply to specific products and they expire and must be renewed. The provisions under which these variances are granted are specific; in certain cases they restrict sales to government and law enforcement agencies only, under a direct purchase order and under certain procurement procedures, as well as other conditions. Certain Class IIIb infrared laser gun sights and illuminators are restricted by variance for sale in the United States to Federal, State, and local law enforcement agencies only, and may not be sold to individuals. These products are intended for use only by agencies with safety training and property custody programs. Sales to individuals, no matter how well vetted, would effectively reverse the intent of these variances based on custody restrictions alone, creating secondary markets for these devices.
The regulations apply to manufacturers of laser products, not private citizens. I have recently reviewed all of FDA's active gun sight variances and we would not amend any variance issued to current variance holders to allow sales to private (non-government) firms or to individuals. ]"
Additional information concerning compliance of laser products may be found on the FDA web site:
Laser Products and Instruments
)
) means any person engaged in the business of manufacturing, assembling, or importing electronic products. 
