We use Help Scout in our business and have been very happy with it, well worth the cost. Help Scout also has a Shopify add-on that automatically displays your customer's information, pulled from your Shopify store, which looks pretty cool.
Groove looks like another great choice to me.
Help Scout also seems to have a limited free plan which looks like it would get the job done for a small business without much support/staff, and when you need more their paid plans currently start at $15/user/month, which I think is totally reasonable for how great the product is.
Hope this helps!