Hi, I posted this link in another thread and I think it is good enough to post here as well.
CBP Responds to HSToday.us FOIA Request on Private Letter Opening - HSToday.us
In my opinion what is of interest to you in this article are the last several paragraphs that describe why customs might decide to examine mail.
“Past practice indicates that the following circumstances (which are illustrative and not exhaustive) provide ‘reasonable cause to suspect’ and permit the opening of sealed letter class mail without a search warrant or authorization of the sender or addressee.
“A detector dog has alerted to the presence of narcotics or explosives in a specific mail article;
“X-ray fluoroscope examination indicates the presence of merchandise or contraband;
“The weight, shape, feel, or sound of the mail article or its contents may indicate that merchandise or contraband (e.g., a hard object which may be jewelry, a stack of paper which may be counterfeit money, or coins) could be in the mail article. Contents of a mail article which may feel lumpy, powdery, or spongy may, for example, indicate the presence of narcotics;
“Information from a source previously shown to be reliable indicates that an identifiable mail article contains merchandise or contraband;
“The mail article is insured;
“The mail article is a box, carton, or wrapper other than a thin envelope;
“The sender or addressee of the mail article is known to be fictitious.”
I imagine they might also randomly select mail. However, assuming your mail is not selected randomly, or found by some other means e.g. dog etc., and looks like a simple letter, then I would think that more often then not you should recieve your mail. (Unless of course customs is by some miracle the only efficient, un-broke, government agency in existence. An impression left by many paranoids cruising these boards, and something maybe more then one seedbank might want you to believe (now who's paranoid?) buwahaha)