San Juan to Vieques flight prices

I’m trying to figure out the best way to get to Vieques in April.

Originally I was planning to rent a car and stay in San Juan (I arrive on the 7th) for a couple days, then take the ferry with my rental car to Vieques. But I’ve read that you cannot book cargo ferries online, only day of at the kiosk…I’m not sure I want to risk that. So then I thought I could wait on renting a car, use Uber around San Juan then fly to/from Vieques and rent a car after returning back to the main island. I just looked at flights and they’re $350 minimum…is that normal?!

Do y’all have any suggestions? I’m planning on April 9-11 in Vieques.