I stayed at Pop Century on WDW grounds in March of this year & went to Universal Studios 2 days. I ended up using Mears Transportation service & had fairly good luck with it. They charge $15.00 round trip. You can call a day or two ahead of time & tell them when you would like to be picked up. (They pick up every hour.) This is a direct transport from your hotel to Universal Studios.
When we did it, they were on time, we paid the driver & then he gave us a voucher for our return. It takes about 10 to 15 minutes, depending on where your hotel is exactly. The nice thing with Mears is you can specify your pick up time when you first make your reservation or you can call that day while at the park (at least 2 hours ahead) & tell them when you'll be waiting. This came in handy as we often stayed later then we originally planned & could adjust our times accordingly.
Disney hotels are familiar with Mears & will assist you if you have any issues with your shuttles. To make reservations once you arrive at your resort, simply call: (407) 423-5566 & say you'd like to make an Attraction Transport reservation. From there it's pretty simple.
To take a Cab to USF is about $30 one way, & the city bus takes about 20 to 35 minutes with the transfer. I highly enjoyed Mears & felt that same "security" that I do with Disney Transporation. Good luck!