If we did not meet minimum do our posts get rolled over into nextweek payments or do they never get credited?
Only for the first week. So if you join on a Wednesday, you will have until about a week and a half to reach the 20 post minimum limit. From your second week onwards, any posts that don't reach the 20 post limit will cease to roll over.
Do you know what time time frame/range is and when the weekly total is calculated?
(Eg. From Monday to Sunday, calculated on Sunday at midnight.)