I don't think the PR manager makes a big difference in merchandise. The reason that your merchandise went from 20k to 34k is primarily because:
1. You got 3 additional draftees which boost your merchandise.
2. You promoted which increases your merchandise.
I think a PR manager is only really useful if you are a losing team (i.e. lose a lot of matches) and so the PR manager will still get you slightly better attendance. However, if you are winning almost every game, I don't think you will see much effect from the PR manager.