Up & Away Games
Or take a turn to discard your contracts and draw new ones. That's a reference to a different way of playing, actually, a different set of rules.
The real answer is that experienced players count up the amount of cash they need to spend to accomplish a particular delivery plus the amount they need in case things go wrong before they can deliver. If the delivery route includes three river crossings they will save 9 (or whatever it costs) for this contingency. If the delivery requires running on the other player's line,t hey will save this amount. If including the contingency amount they cannot afford to make that delivery, they do not do it and instead choose another demand.