The surprising thing in this case, is that customs sent the letter in the first place. I can not think of any reason for them to do that other than to invite you to incriminate yourself.
According to the post by jmollner, a package of viagra from overseas is illegal, even with a prescription. There is nothing you could tell customs to make it ok, even if you had a prescription.
However, as it stands, you could deny you ordered it. Customs simply invited you to make their lives easier, hoping you would just fuck yourself.
I say it is surprising because I didn’t think customs bothered with these small quantities of non addictive drugs.
I suspect that the lobbying clout of Pfizer and the others has resulted in increased pressure to address this patent infringement issue.