The letter from DHS's Customs is pretty much the same as the old ones from Treasury's Customs. I've seen posts from people who claimed to have gotten a seizure notice from DEA. Now that would be scary. But somehow i doubt that they'd be so open about it, and the posts were probably mistaken about who the letter came from.
Once a federal agency sends you an official letter, they can keep a file on you and you often have the right to get copies of it. But your Privacy Act rights to get copies of your government files would not apply to an ongoing criminal investigation. Regular seizure notices by Customs and FDA are *not* a part of criminal investigations, so anyone could do a Privacy Act request for their own seizure-notice files kept by Customs. There's a Privacy Act request form-letter you can find on the web, and you have to have it notarized and promise to pay for copying expenses ... and they will send you your file. Parts of the file can be blacked out. You'd have to do it for each individual government agency: FDA and Customs are very separate.
If the DEA were interested in the mail you were receiving, i reallly doubt they'd send you a seizure letter (or give you your file). Maybe they'd show up at your door and threaten/intimidate/bully-their-way-in ... but no letters in the mail unless it was a subpeona.
One major change in import/export law since 9/11 is that Customs is now allowed to search the outgoing as well as the incoming stuff. I don't think anything else in the law has changed with regard to mail searches by Customs.