Short answer: Journal submission support cost depends on how much preparation is required before the author submits. Formatting-only support is different from a service that also checks required files, cover letters, declarations, figures, supplementary material, and journal-system requirements.
Ask for a written list of deliverables and confirm which tasks remain the author’s responsibility. Authors should keep control of their accounts, declarations, approvals, authorship information, and final submission decisions.
If the target journal changes, additional formatting or document preparation may be needed. Clarify whether that is included or quoted separately.
A submission service can reduce avoidable administrative errors, but it cannot guarantee editorial screening, peer review, acceptance, or publication speed.