I would just try and avoid simple trade altogether.
Plot at Empire is generally about making Profound Decisions. Deciding whether to trade 20 tempest jade for 40 True Vervain isn’t a Profound Decision - it’s a purely economic one, it’s either worth it or it’s not. As such, it’s a lot less interesting to the Plot team if a Winged Messenger is sent purely talking about trading one resource for another. All of that is already available on the field, and is presumably available in the foreign country as well.
But offering to do something beyond that can get very interesting. Perhaps that involves pulling levers only available in Anvil, like Senate motions, Synod judgements or Conclave declarations. Perhaps that involves PvP (political or otherwise) with PCs that the foreign nation dislikes. Perhaps that involves dealing in truly rare resources, like True Liao (which is very illegal to trade to foreigners!).
To look at it another way - you’re contacted out of the blue by someone from a foreign nation. You have no easy way to check who this person is or how reliable they are. If they start to talk about doing things extremely in your interests - that’s likely to hook you more than offers to trade, which is probably a large part of your life already (if you’re talking about foreign ambassadors, at least).