It’s quite difficult building a service like that, because it’d have to deal with so many different scenarios depending on the service/product type. On Quaderno.io we opted to use the standard VAT rate by default to automatically create the invoices, and let the customer change it if necessary.
In your case I see two possible solutions:
- your users specify the VAT of every product
- your users specify the type/category of every product, and then your app automatically calculates the VAT rate
I think the former is better, because most of the user will need to charge just one VAT rate, and on the other hand, your app won’t have to handle all the possible scenarios and VAT updates, which can be a completely mess.