You know, I've actually never heard of PlanetExpress before, which is somewhat surprising, since part of my job is to know about our competition!
Here's what I can see:
Overall, it seems like these folks definitely know what they're doing. I will say I'm a little bit concerned that I can't read any testimonials.
Here comes the shameless plug, but it's true -- if you go visit us over at https://www.shipmonk.com, you can see reviews from "easily Google-able" clients. In general, I strongly recommend two things:
Good luck out there!
Hi ShipRelay here! Check out our https://apps.shopify.com/shiprelay-fulfillment-center and this video of our carrier calculated rates in action here as we could also be a good fulfillment partner fit.