The Investment step is the first step of a turn. This way, a player has plenty of time in the previous player's turn to think about investing and what to buy.
(In Merkator, you can theoretically plan your whole turn while the player to your right takes his/her turn).
If a player skips the Investment step and chooses a location while the forced investments rule is in effect, that player immediately has to sell Contracts for coins and gets no chance to buy Bonus cards or Building cards this turn.
If a player travels to a Major location, it is easy to check if the distribution of goods has been overlooked. Check the adjacent Major locations.
If any of those locations has no goods, something is wrong. (Two connected Major locations can never be emptied of goods at the same time).
If a player travels to a Minor location, also check the Major locations to see if the distribution of goods was missed. For example, if the player traveled to Bohemia (3+), there may never be exactly 3 cubes in any of the Major locations afterwards.
Players joining the travel to one of the Major locations don't have to wait for the active player to finish his actions. The can fulfill their Contracts immediately, as Major locations never have 2 Contracts of the same value.
End of turn
Players should announce that they're done. Players joining the travel should declare this early.
Building and Bonus cards
Most Building cards score fewer VPs than players paid coins for them. But as surplus Contracts have to be sold anyways during the regular rounds anway, and score just half their coin value in the final round, investing in Buildings is still profitable.
To invest in Bonus cards is normally profitable if you get goods from that card at least twice.