A social media manager runs the account rather than making a single piece: what goes out, when, in what order, and what happens in the comments afterwards. It is an ongoing job, and it is priced like one.
Almost always a monthly retainer, and usually tiered by how much original content is included. The cheapest tier is normally scheduling and replying; the expensive part is anyone actually making things.
Be clear about who is producing the content. A retainer that covers posting but not filming is a perfectly good arrangement, but only if you have already worked out where the videos come from.
You say what you are trying to achieve rather than what to buy. Lucy shapes it into a brief, prices it against what people here actually charge, and shows you the creators who can do it. You agree a figure before anything starts. The money is held until you approve the work, and Lucy takes 15% of the creator’s fee — never a fee from you, and never anything on approved expenses.