Yes, you can. Legally, you are allowed to use one or multiple programs to manage your business, as long as invoices are issued sequentially in different series. If you are a Self-Employed Worker (Trabalhador Independente, TI) or a Sole Proprietor (Empresário em Nome Individual, ENI) and currently use the Portuguese Tax Portal (Portal das Finanças), you can legally issue invoices with FIZ Invoicing if you find it more convenient.
If you already use an invoicing system and want to start working with FIZ, simply configure the communication with the Portuguese Tax Authority (Autoridade Tributária, AT) in your FIZ profile. FIZ will generate new series, allowing you to continue issuing invoices within the newly created sequences.
If you are fully satisfied with FIZ’s features, you can transition entirely to FIZ without waiting for your current software subscription to expire.
Please note that if you previously issued invoices through the Portuguese Tax Portal (Portal das Finanças) and switched to FIZ, the invoices you issue will still be available in your e-Fatura system on the portal. For more details, check the article "How can I view my documents on Portuguese Tax Portal (Portal das Finanças)?"