There's a lot at 1323 South Capitol Street SW that I would highly recommend. But it's $39 if you use Parking Panda/Spot Hero to reserve (and I wouldn't recommend not reserving it ahead of time).

The pros are that it's RIGHT across from the stadium, so you park and you're right there. Similarly, when it's time to go you're right there. More importantly for you, though, it's right on South Capitol Street, and when leaving the lot you just take a right on South Cap and follow the signs to 295 and you're on the BW Parkway within five minutes. Easy-peasy.

That said, I would at least think about leaving the game an inning early to beat the traffic. If the game ends at 3 hours, you're looking at 4pm. You *should* be able to get to BWI from the stadium in two hours during rush hour traffic, but it depends on your comfort level. Especially with having to return a rental car, you might want the extra time. It's one of those things that by leaving 10 minutes early you save 45 minutes. But I think unless it's a long game, you should have time to get from this lot to BWI since you're getting on 295 almost immediately.

Put the lot address into a map and you'll see what I mean with the exit. The only trick to the lot is that it's only accessible when going the one way on South Capitol Street. But even if you approach in the wrong direction, you can make a U-turn on N Street (technically three lefts, but it gets the job done).