I've done that several times and will probably do it again in Sept. I always buy way in advance. Then, the day before we want to go to US/IoA, we call Mears and make our pickup time. We usually leave WDW by 7:30. We've made at least one or two addtl stops, but nothing excessive.
But, going back can be longer. We usually give Mears a call about two hours before we want to leave. They then tell us what time to be at the bus stop. More often than not, I have gotten to the bus stop and been put in a van, not a bus. But, that van makes multiple stops...so it takes longer to get back to WDW. So, plan accordingly.