In the case of different VAT rates on the same invoice, you can use the 'Line Items' functionality. It splits the invoice into individual items. Please select a relevant 'tax code' from dropdown for each line before exporting to Xero (Datamolino will remember these settings for the future).
 
Note: 

Datamolino exports only the amount in bold (in this case only NET) and Xero calculates the rest based on the tax code you have chosen. Therefore, there is no need for completing other figures (Tax + Total).

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