I think the idea is good, but a Moat that hands out one-shot Labs for $4 sounds awful. It might be okay, given how good Offering is for you, but it LOOKS terrible. Maybe improve the card that gives out offerings and make offering itself a bit weaker (for the player who gave it out)?
Or maybe just make Offerings effect smaller - maybe a cantrip that gives you and the other player a card? There's still a good reason to play it, as only playing it removes it from your deck, and giving somebody +1 card feels more acceptable than giving him +2 cards.
Either way, i think ii'd like a +1 card version. Courier would be like a weaker Witch that gives out Confusions if nobody played Offerings, and a Moat with delayed Lab effect if they did.