The letter is almost certainly going to be tampered with. She gave a copy of her seal to her worst enemy, and I'm sure that he knows how to open the envelope without breaking the seal.
I wonder why he was there in the first place though.
@PandaBambi
The nobility of this comic are based more on Asian standards and etiquette than European. You can see this in how each member of the imperial family own their own palace or in their communication style, which is much more Asian. The etiquette of this style of nobility is heavily entrenched and ritualized, so their schedule is rigid and predetermined even for things like a couple having sex.
This is also why she can't just say to Cesar 'ur dad told me to gtfo'.