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Shippo.com service levels, for example, are all lower case, an example of a shippo carrier code and level would be:
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Note that the “Carrier” field is a combo box - not just a drop-down. While there are drop-down options for DHL, Fedex, and UPS, you can also type anything into this field. This allows entry of all lower-case entries, necessary for the different shipping carriers for Shippo.com.
On the shipping method screen, if the shipping method has “shippo” selected, all lower-case carrier codes show as options; the desired shipping level can be selected on the shipping method screen:
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