You're safe enough booking through a major US ticketing organisation such as ticketmaster ticketmaster.com/mlb, and paying with a credit card gives you an added layer of protection.

The figures I'm getting from major car hirer operators suggest a one-way hire fee of around  $350. I'm going to the Australian websites of the hire car operators themselves rather than third party sites and this offers a fair degree of security that things will go the way they're supposed to when you front up to the car-hire desk. I've also logged on using a Virtual Private Network to disguise my location to see if the price is any different for a US-based hirer and it's actually more expensive. There's really no way to avoid this one-way fee but split between the four of you, it doesn't seem too excessive. Once you get to the first of your east coast cities you probably want to return the vehicle, in either Boston or New York it's more trouble than it's worth, and you're better off getting trains between the two cities.

You need insurance cover. In the US you may be liable for the entire replacement cost of the vehicle if it's severely damaged. You can buy collision damage waiver to cover yourselves against this at the car hire desk but it's very expensive. There are a couple of third-party sites such as RentalCover.com  that provide cover at a much lower price.

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