It depends on the seller and the reason the package was lost. There are only four international sellers that I trust overseas.

If your seller is not using EMS (Priority Express), then the package is more likely to be held in Customs for however long Customs wants to take. With EMS, the package should not take more than a week to arrive; if it does, contact USPS and then the Customs office the package is at or destined for.

If your seller is improperly declaring items (falsifying number of plants, calling plants household goods, not properly documenting the price of the goods, etc.), Customs can very well destroy the package.

If your seller is sending plants without phytosanitary certificates (and you will know if they are, because they will charge you for them), Customs has every right to destroy the package.

If your seller is sending the plants in dirt or any other non-approved media, or if the plants have been found to be infested with bacteria, fungus or pests upon an USDA inspection (packages are randomly pulled for USDA inspections), your package is likely to be destroyed.

If your seller cannot be bothered to double and triple check that the tracking number they gave you is correct, then you might not want to do business with them.

If you were able to track your packages, meaning you were able to see that they arrived at a US airport, then were "Inbound into Customs", then your tracking numbers were valid. Who is saying the tracking numbers were invalid? The parcel service used? Customs? The seller?

If the tracking number was invalid, how do you know it got stuck at Customs? Did they call you? Did you call them? How did they find the packages, if they did?

I would never do business with a seller who cannot be bothered to follow proper procedure regarding US import laws, nor wpuld I do business with a seller who cannot be bothered to assist me, should something go wrong, especially if they were at fault.