Like Disney, when you use the tickets depends on the crowd seasons. The Universal website shows clearly what is value and not value season ticket times.
And, express pass is day, and park(s) specific, with no refunds.
While most recommend buying at park, to see if you need it, as it is sold everywhere in both parks. While it does not generally change in price, it may as Universal will increase if demand is there. So if you are absolutely sure you want that park to park express, buy it.
Oh, and like almost any park, a one day is the most expensive way to buy.
I would buy from Undercovertourist, as their price includes all taxes, and every little bit of savings is a win in my book.
I hope this helps.