I'll tell you, RepsDirect is pretty good. We fulfill for > 100 eCommerce stores, and I'd say roughly one-fifth outsource their support.
To be completely honest, though, the ones that outsource support also end up having the most complaints regarding refunds, returns, etc.
Hire someone part-time. You'll have direct control over the entire process when you need to intercede, and the communication chain will be much easier. If you are also outsourcing fulfillment, this is especially important.
We've been using Proonto's customer service platform for 18 months now and I have to say it's incredible. The combination of both technology they provide and a team of dedicated product experts they built for us has been proven to boost sales and customer satisfaction. You can easily install the platform from: apps.shopify.com/proonto. It's straight forward from there and the company team is super helpful.