This is not airline-specific, so will relocate to the appropriate Forum. /Moderator

Your $800 duty exemption is only if you have been out of the country 48 hours or longer, and you have not been abroad within one month (in which case your duty exemption is only $200). You are entitled a $1,600 exemption if you are returning directly or indirectly from a U S Insular Possession (e.g. Guam, USVI, American Samoa.) And many items purchased in countries covered by the Generalized System of Preferences (GSP) may not be dutiable at all. (OTOH, some items form some countries are dutiable - at 100% rates!)

Also note - the first $1,000 of excess is generally dutiable at a flat rate of 1.5~3.0%, and further excess will be taxed at the rates for those kinds of goods. USCBP is actually and generally pretty good about covering your highest duty purchases in the $800 exemption, and then in the $1,000 flat duty exemption.

Also note - items mailed or shipped back separately DO fall under your returning exemption limit, as do repairs (e.g. on cameras, etc.) (However, you can send / mail $100 in gifts per person daily to others in the USA duty and tax free; $200 from insular possessions.)

These are very general guidelines and should not be relied upon. Your authoritative source is U S Customs and Border Protection, and you can download the current "Know Before You Go" brochure here.

I can tell you this from over a half century of experience in dealing with USCBP / Customs, from many countries with many goods, including having permits for animals, plants and other goods: always disclose fully; they will often cut you considerable slack.

The consequences for not disclosing and attempting to conceal can be pretty dire, and I have seen all kinds of people with shoes and clothes they purchased but soiled and wore, goods they attempted to wear or conceal, etc. etc. and there is no tolerance for any of it - they will often be merciless, and if you belong to a GOES / Trusted Traveler program, you will lose that privilege. Oh, they will likely forward records to your state, which like California, may communicate with you and charge you a "use tax" if they have a sales tax, just to add insult to (self-inflicted) injury.