Alamo and National use the same programs. It's one of the better rental car SunPass programs. I've used it with National when our trip included Southern Florida. You don't get a transponder because the system is based on license plates.
You don't have to opt in or opt out ahead of time. If you use a SunPass lane (whether on purpose or accidentally), you've opted in for that day. The toll(s) and fee(s) will hit your credit car a couple of weeks after you return you car as separate charge.
If you pay all your tolls in cash, you don't incur a charge on your credit card.
If you're only traveling between Orlando Airport and and the tourist corridor, paying cash tolls is cheap and easy. On the northern route, there are two toll plaza each way. On the southern route, there are two plazas each way if you get off at exit 6 (and get on the same place when you head back to the airport).
In either case, pay attention and keep right. There are attended lanes that provide change and receipts. You don't need a pocket full of quarters. (If you stay on the southern route beyond exit 6, you'll have to pay a cash toll with quarters.) If you only use attended lanes, always get a receipt. That way, you have proof that you paid.
Once you're in the tourist area, you can visit all parks at WDW, Universal, and SeaWorld without encountering toll plazas.
If you're heading to Southern Florida, it's a different story. The Miami area has a bewildering network of toll locations, and many are unattended.
People can and do get their own SunPass transponders, which they can use year after year. But it's not without risk. The license plate on your rental car is still registered to SunPass as belonging to your rental car company. If a reader fails to pick up your transponder, you'll still be charged with the toll and fee on your credit card.
