Details, Details, indeed.
Yes, there is the 35 minute rule between shops. It is measured from the time stamp on the receipt of the first post office to the entry of the next one. There are also restrictions in that you have to enter 35 minutes after opening and 35 minutes before closing. If that location closes for lunch then the lunch break counts as opening and closing times for this rule.
As for doing more than one in a day, yes you can. The limit is four per day and 20 per week. Just observe the 35 minute rule in all it's permutations.
As for doing more than one in a day at the same post office, you may do a "C" and either an "A" or "B" shop at the same post office in the same day. However, there needs to be a two hour difference between them. I also seem to remember they need to be in different segments of the day. Have a look at the instructions for that detail.
Have fun!
Happily shopping Rhode Island and nearby Massachusetts and Connecticut