Validating early in the pipeline is smart, since a rejected XRechnung usually surfaces weeks later when the customer's accounting system bounces it. One thing worth adding is keeping the validator report alongside each sent invoice, because the KoSIT rules get updated and you may need to prove an invoice was valid on the day it went out. It would also help to note which of the BR-DE rules trip people up most often, like the missing buyer reference.
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Validating early in the pipeline is smart, since a rejected XRechnung usually surfaces weeks later when the customer's accounting system bounces it. One thing worth adding is keeping the validator report alongside each sent invoice, because the KoSIT rules get updated and you may need to prove an invoice was valid on the day it went out. It would also help to note which of the BR-DE rules trip people up most often, like the missing buyer reference.