Letter
The burying part was easy. What he needed to be more careful of was the envelope that he would send to Guildmaster Bradt that would inform him of the location of the esoteric resource supplies that he had just buried on the other side of the nation.
This was a bit more dangerous since Rui needed to make sure that he didn't by mistake clue Guildmaster Bradt in on his location or identity.
He used the most generic and commonly available and supplied kinds of paper and envelopes with the most common kind of cheap pen. This way, there ought to be no real way Guildmaster Bradt could possibly track him based on the kind of paper, pen, or envelope used. Some might say Rui was being paranoid, but Rui knew better than to assume that Guildmaster wouldn't at least thoroughly look into this avenue once.