I would definitely mail your seeds back. Way easier than trying to hand carry. I have come in through Seattle in the past with seeds in luggage, but maybe that has changed?
As far as plants, there are a lot of regulations, but you CAN bring in up to 12 plants back in your luggage WITHOUT any permit, provided they are not on the "prohibited" list, do not require special procedures, are of a certain size, have a phytosanitary certificate, and a few more rules. They can't be in soil (I love the polymer crystals because when you get home, you can plant the whole thing, crystals and all). The USDA has a whole list of things they can be planted in.
The size and age of plants allowed varies, but you can check all that out on the USDA APHIS site at
https://www.aphis.usda.gov/aphis/ourfocus/planthealth/import-information/permits/plants-and-plant-products-permits/plants-for-planting/ct_q37_protocolsIt can be pretty complex, but if you just have one or two plants in mind, and can find a nursery that does permits etc, it may be worth it. I have read up on it, but never actually tried it.
Hope that helps!
Carolyn