Universal started charging for the Express passes in 2005 I think...it was after I moved here in 2004 I know that much. The reason I always heard was the Express lines were too long with the free EP so they took out the Express machines and starting charging people for Express access.
There are two passes...the 1 day 1 park or the 1 day 2 park...Express Plus passes are good for one time through the Express lane on all attractions except the Pteranodon Flyers at IOA. Depending on when you're going, you may not need the EP's and since you've got the seven day pass, I wouldn't fool with it right now. You can always purchase the EP's at either park once you see how the lines are. Express passes are date specific and non-refundable so don't buy them until you're absolutely sure you're going to need them.
If your not staying on-site,you have to buy them daily.Prices will vary depending on your dates and what Universal will consider busy.The ones you buy daily is only good for one entrance per ride for that day.Staying On-site is unlimmeted for length of stay.
Your better off getting to the park at opening and see what the crowd-waite time is before you purchase them.
I remember when Universal did it like DW,and it was crazy.You were better off standing in the regular line.Now with the EP system they got,5-15 min tops to get on the ride.