Answer
Calling the invoice print action directly from a workflow hits an internal savepoint/rollback conflict in the underlying API and isn't a workable path. A Report Rule is the practical alternative: configure a Report Rule filtered on the relevant parameters (like payment terms) so the invoice prints automatically when it's created with 'Print and Create' (rather than just 'Create') selected — that reuses the standard client print path instead of trying to invoke the print action programmatically from a workflow.