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Read onlyShipstation will start charging users and extra $30 - $100/month (depends on your plan) to use external shipping accounts including Amazon Buy Shipping, starting next month. This is on top of the subscription fees increasing next month too.
Shipstation's parent company also owns the following shipping services (likely to implement something similar), so take that into account if you're looking for a new provider.
-ShippingEasy
-ShipWorks
-ShipEngine
-shipsi
It is so disappointing that they are charging this much for using the Amazon Buy Shipping program. It almost doubled our fees for using Shipstation.
We are now evaluating other providers but there does not appear to be many that even supports Amazon Buys Shipping connector.