"What should you do if you receive a letter from Customs"
Trash it, its just a "love letter" from them telling you they seized you're meds. If its controlled than its illegal to import in and I would assume challenging the seizure would result in admitting guilt.
Now if it was allergy meds or something like that that got seized then I guess you could challenge it.
Most likely you get a second letter month later if you don't challenge it saying that the meds/articles have been forfeited.