The marketing contractor charged me. The way it worked was that if a new email or WhatsApp message or Instagram PM came in, he would get notified and work on it. And he measured the time for each task and then billed me for his time. Since it was usually only a few minutes for each work task, that only summed up to a few hundred $ per month.
As one example of the kind of tasks he handled:
Prospective customers could send in their measurements via WhatsApp and he would then look up into our internal measurement tables to determine which bikini size to order.
Or we would get collaboration invites from Instagram Influencers via PM or E-Mail. He would check their follower counts, bot ratio, etc. and then use an (internal secret) Google Docs to calculate the estimated value for us. He'd then use that to either decline or determine how generous our promotional gift would be.
Nowadays, the concept is called VA = Virtual Assistant, for example like what https://timesvr.com/ offers.