Test PayPal Payments Pro before signing up for a PayPal Pro account?

Is there a way to test the paypal payments pro service before signing into an agreement with them and being billed? I'm thinking I could with the sandbox but I don't know for sure. If the sandbox is the only way, how much work is it to go from that to the real deal when the site is ready??

Sounds like an excellent question for the folks at PayPal. https://www.paypal.com

Is anyone here doing this already that can offer some tips?

What exactly do you need to "test"?

The sandbox is for programmers who are trying to connect the shopping cart to PayPal. But that's already done for you in Zen Cart. So from that context there's no need for a storeowner to bother with the sandbox. Besides, there are a whole lotta hoops to jump through in order to actually use the sandbox, and it's rare that a storeowner would care to lose the hair over it.

I want to run some test transactions to make sure that everything is set up correctly. Do I not need to worry about this?

Once you have your account approved for the Payments Pro feature, if you follow the setup instructions and then run a couple transactions, you should be good.

My items are all $300 + though. Will it be a problem to use a credit card and just refund the money? I'm not sure how WPP handles returns and fees..

I would plan to charge and refund the transaction, regardless of the amount. It's not uncommon to temporarily set the price of an item to just $1.00 for the test and then put it back, or to create a temporary product for $1.00 and then delete it after testing (or keep it around but just disabled so it can be used for future tests, since it's always a good idea to buy something from your store as a test every couple months anyway!).