If you need to add Sales Tax to your customers documents, you need to make sure the customer is set to be 'VAT Applicable'.
If you need to remove Sales Tax to your customers documents, you need to make sure the customer is NOT set to be 'VAT Applicable'.
Tip: The VAT rate will be calculated based on your Country set in your Company Details. You can either apply VAT or not.
If you need to change the VAT rate on the wholesale order because you are registered (have fiscal representation) in more than one country, you'd need an Umbrella System which enables you to have multiple entities/divisions, each with its country set in your Company Settings.
In the Customer record select [VAT Applicable] from the Payment Details Tab
Sales Tax based on your National Rate will now be added to all future orders, proforma invoices and invoices for this customer, at your country's rate. For existing Sales Orders, Proformas and Invoices you will need to Update Customer Fields.
For EU clients - you can check if the VAT number is valid. Enter the tax number with the country prefix, then click 'EU VAT Verify Online' button for Zedonk to verify the VAT Number.