seems to be mostly random inspections
some packages/letters are opened and resealed with the infamous green tape(indicates opened by customs
there may be some inspection devices, less knowledgeable about that
of course, it's not completely random, some things will red flag an item, there's thread around here somewhere on that
packages are more likely to be opened, there are fewer packages than letters and a package has the greater potential for carrying 'larger' illegal items
for small seed orders, customs just replaces them with a letter saying you have attempted to import a forbidden item
consequences are unlikely, unless you are in one of the bad states which seem to focus on seeds - Indiana comes to mind