How do I create a new invoice customer for Wave?

You can create an invoice customer from within taxomate.

To create a new customer head over to Settings > Connections or the payments tab in Quick Setup.

Select the invoice customer dropdown and then Create New.
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You can choose any name you would like (e.g., Amazon.com).

❗ IMPORTANT: For currency, you are going to need to select the currency of the marketplace you are creating the customer for. For example. Amazon.com would be USD, Amazon.co.uk would be GBP, Amazon.de would be EUR, etc.