The fare rules I read did not address payment due dates. I will examine them again.

For anyone who might be interested, a cruise board where I asked - there are numerous TAs on that board - opined that these are likely consolidator fares the TA is using. That along with them likely running quite a volume of business through the consolidator allows them to provide this float to me.

As I do a lot of business with this TA I will just ask. She should be used to my weird questions by now.