Why are several invoices generated if I have selected delivery notes from the same customer?

It may be because you have registered in the Customers section, with the same name and VAT number the same customer, so that when generating the documents you have indicated different customer references.

For that reason, several invoices are generated, since the program does not recognize the same customer reference for those documents.

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Access the delivery notes, press Edit and indicate the same customer reference for those documents.

You can now generate the invoice for multiple delivery notes!