We also have the removable suction cup kind, set to add money in $20 increments. We make both car and RV trips to FL, three or four times a year to vacation and see friends & family. It's flexible - you can add or subtract vehicles to the account online as needed.
The website lists places where you can purchase either type, sticker or removable, such as grocery stores. I think it takes 24 hours to activate, at least for the removable one.
The main reason I like Sunpass is that the turnpike headed south to avoid Miami & I-95 no longer has toll booths, at least some parts, so the only options are a Sunpass or a mailed bill. It can take weeks for the bill to come and we might be off on another long trip by the time it turns up.