Rulings

For Riveteers Charm's first mode, if the target opponent controls more than one creature or planeswalker with the highest mana value among creatures and planeswalkers they control, that player chooses which one to sacrifice as Riveteers Charm resolves.
For the second mode, you must pay all costs and follow all normal timing rules for cards played this way. For example, you may play lands this way only during your main phase while no spells or abilities are on the stack and only if you haven't played a land yet this turn.