You certainly can. Apart from a range of toxic chemicals that come to mind. there are also radioactive isotopes with specific half-lives that would meet the requirements. Then there are various bacterial and viral agents that would meet the requirements.
As far as posting, writing and handling a toxic agent is concerned, why involve the Royal Mail etc.? If an innocent looking letter dropped on my doormat anytime in a given morning, I'd think it had been delivered by Royal Mail instinctively. Wouldn't we all? I do still sometimes receive mail after the first lot has dropped on the doormat. Posting is therefore not a problem. As far as handling/writing goes, if the malevolent individual is that determined to do his/her worse and has gone to that extent to obtain the necessary stuff, these handling and writing would be hardly rocket science for him or her would it?