You can create a template per customer, but it can not be assigned to the customer, as you said you have to choose the template when creating an invoice or sales receipt

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Resolution for 'assign invoice issue: 'assign invoice template per customer' available: Yes.

The assign invoice issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the assign invoice issue yourself by looking for a resolution described in the article. If it is a complex assign invoice issue or you are unable to solve the issue, you may contact us by clicking here or by using other assign invoice support options.

For more information about resolving issues related to assign invoice in Quickbooks/QBO or to get support for the issue 'assign invoice template per customer', click the link below: