If you want to send an order from a non-Amazon channel to be fulfilled by FBA, and wish to specify the shipping type, then the shipping priority used by the FBA centre will be based on three keywords in the names of Linnworks Postal Services:
By default, Linnworks will set the Shipping Speed Category to Standard for the FBA order. If the Postal Service assigned to the order contains the word "Expedited" - Linnworks will rewrite the data sent to FBA to "Expedited"; if it contains "Priority" - then "Priority" will be sent to the FBA as well.
For example, a service with the name "International Priority Shipping" contains "Priority", so Amazon FBA will use the Priority service when they ship the item. Any service name without those two keywords will be by default sent as Standard.
You can either set order shipping services manually, use Shipping Mapping to assign the services based on the services buyers choose on the channel, or set up Rules Engine rules to assign services based on any set criteria.