Board Game Arena

In playing through the ages a colony came up for auction and I was trying to use a bonus card on it, but it just kept saying that the card was unauthorized. Are these cards used another way, or is there some specific circumstance required for using it? The rules seem to say that they can just be used to bid.

You're supposed to sacrifice at least one unit in order to bid, no matter how many bonus cards you have.

I have cartography and want to bid 2 (single warrior + cartography bonus). Do I bid to "sacrifice" one point of strength or two?