I have included the clients' tax id's in their client cards under sales tax registration. When I issue invoice to client, their tax number does not show on sales invoice.

Thank you for reaching out to the Community, Konstant. Let me clarify more about sales invoices and custom form styles.

In QuickBooks Online (QBO), there isn't a built-in feature to automatically display your client's tax number on invoices; only your VAT ID can be shown.

Although QBO does not automatically show your client's tax number on invoices, you can utilize the custom fields feature to manually add this information to each invoice. Here's how you can set it up:

1. Go to the Gear icon.
2. Under Lists, select Custom Field.
3. Click Add Field.
4. Provide a name of your choice and check the box for All Sales Form.
5. Hit Save.

Additionally, you can use the following link to access a guide that will assist you in generating a custom report of your financials. This resource can help streamline your reporting process.

If you have any additional questions or need assistance with sales forms or any other features in QuickBooks Online, please feel free to respond here. I am here to help and will reply as quickly as possible. Your satisfaction and clarity in using the platform are my top priorities.