This will be a lot to take in.
All galaxy accounts start with 3 post credits. If you lose all your post credits you won't be able to post. You can gift post credits to others, but only up to 2 at a time. You can only gift credits to others if you have more than 2 post credits, and they have 0 post credits.
Your messages must have in them a set of answers contained in spoilers, as shown below
A.
B.
C.
D.
All the answers must be 2,1,-2,-1, but you can put them in any order you like
However, you must complete the last post's answers first. In your post, you must state whether you gifted 1 or 2 post credits to another account or not. You must also state your new post credit amount. The first option you pick is how many post credits you lose before you make the post. If you drop to 0 post credits before you post, the post you make will be your last post. You cannot play the multiple choice until someone else gifts you post credits so that you're able to lose post credits again.
Unlike in post economy, you don't lose post credits when posting; you can only lose them by picking the wrong answer in the multiple choices, or by gifting post credits. If you happen to pick -2 while you have only 1 post credit, your post credits only decrease to 0; you cannot have negative post credits.
You cannot post back-to-back with another post that came from you.

Example: user713, who in this scenario is the second to post, picks the option in this post that takes away a post credit. For the sake of example, lets say they started with 4 post credits. They still have 3 post credits leftover; therefore, they can still post. In their post, they set up the multiple choice with -1,2,-2,1 as the sequence of answers. They state they gifted a credit to user534, who, for the sake of example, has started with 0 credits. user713 is only able to do this because user534 had 0 credits, and user713 had >2 credits after playing multiple choice. They state they have 2 post credits left as of making this post. They then bash their head on the keyboard a few times, miraculously only hitting numbers and letters. If instead user713 started with, say, 2 post credits, they would've been left with only 1 post credit after picking the -1 answer in this post, rendering them unable to gift post credits to others. Alternatively, if user534 started with 3 post credits, they wouldn't have been able to be gifted the post credit from user713. If instead user713 started with 1 post credit, they would've been left with only 0 post credit after picking the -1 answer from this post. Still unable to gift, they make their post and then wait for someone else to gift them 1 or 2 post credits to be able to play the multiple choice from the last post and be able to post again.