There are many shipping providers but in our opinion it depends on how many parcels you will be mailing. Since most users will be using USPS for shipping I will focus on that and the providers we support.
Some things to consider are, amount of parcels you will be sending per month, what features the provider has and will you be using any of these features, monthly cost of the service to have these features.
If you are starting out and not shipping more than 50 parcels a month, Shipping Easy is a good start. There free tier allows up to 50 free orders. The next tier starts at 29.95 and have the most features. You have the ability to send your orders to Shipping Easy but you must use the Shipping Easy dashboard to create your shipping labels.
If shipping less than 300 parcels a month, you might consider using Shippo. They have a flat rate of 5 cents per label + postage. They only focus on printing lables and only have basic shipping features. You have the ability to create shipping labels from My Fashion Emporium.
Most have heard of Stamps.com and in our opinion is best for most moderate use shippers. They currently charge 15.95 per month+postage. You have the ability to create shipping labels and buy postage from My Fashion Emporium.
If you know of any other good providers, please let us know and we will check to see if we can add support for them.