Using PEPPOL with Odoo is completely free.

It is free for all Users and Odoo will register your Company automatically when you add your details after activating the feature from Settings:

There are no monthly or transaction charges.

Additional details:

• Peppol registration is free and available in Odoo Community
• You can send Customer Invoices and Credit Notes and receive Vendor Bills and Refunds via Peppol.
• You can send and receive in one of the following supported document formats: BIS Billing 3.0, XRechnung CIUS, NLCIUS.
• The following countries are eligible for Peppol registration in Odoo:Andorra, Albania, Austria, Bosnia and Herzegovina, Belgium, Bulgaria, Switzerland, Cyprus, Czech Republic, Germany, Denmark, Estonia, Spain, Finland, France, United Kingdom, Greece, Croatia, Hungary, Ireland, Iceland, Italy, Liechtenstein, Lithuania, Luxembourg, Latvia, Monaco, Montenegro, North Macedonia, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Sweden, Slovenia, Slovakia, San Marino, Turkey, Holy See (Vatican City State)

from https://www.odoo.com/documentation/18.0/applications/finance/accounting/customer_invoices/electronic_invoicing.html