you read fine - they're really are no rules re. comments...
I agree, but here are some hints:
1) include a reference to which exchange it is related to;
2) include your Photrio user name;
3) include a title that makes it easy to refer to the card; and
4) include additional info if you want to include additional info - either on the card, or in a post in the thread.
All of which makes it easier for the recipients to both keep the cards organized and to make comments.
And some further hints:
When you enter a round, set aside a mid-sized envelope to hold the cards as they are received.
Mark the envelope with something that identifies the round - e.g. "Postcard Exchange #48".
If you can, print a list of all the Photrio user names that you expect to receive cards from on the outside of the envelope.
As you receive cards, comment on them, then mark on them the date you commented. Then mark on the envelope that you have received and commented on those cards,
These hints come from experience - I'be been participating since round 17, and before becoming this organized I had more than one opportunity to get truly confused about several cards lying inter-mixed on my desk.