To sync your Shopify contacts with Holded, go to Settings > Integrations > Shopify, and under "Advanced settings" check the "Create contact" box:

Contacts will be created in Holded through the sales sync, via your Orders and Invoices.

☝🏼 Please note that if the checkbox is unchecked, no contacts will be created and the order will be generated with empty customer information.

Town, province, zip code and country, address, ID (or business name), email and phone number.

Shopify does not collect the NIF because it does not issue invoices; however, in its settings you can change the Company field to DNI/NIF.

To do this, change the "Company Name" option to "Optional or Mandatory" in the Shopify settings. You can then rename the "Optional company label" field to "DNI", for example, and save the changes.

You cannot match Shopify contacts with contacts created manually in Holded, even if they share information such as VAT number or email address. Shopify contacts have their own ID to avoid duplicates.
One way to manage the contacts after the integration is to assign the contact synced from Shopify the invoices prior to the integration, and then delete the old contact created manually in Holded.

You can, however, relate the contacts that come from the integration, by ID, NIF or email, since, from the contact created with the first synchronised invoice, future purchases will be automatically linked.

This is probably because you have unchecked the "Create Contact" checkbox in your Shopify settings (leave it unchecked if you don't want to create contacts for your Shopify orders in Holded).

See how to sync Shopify contacts with Holded here.

No. If the same customer generates multiple orders, only one contact will be created in Holded. The first order placed by a customer will create a contact in Holded, and the following orders generated by the same customer will be linked to the initially created contact.