there are really two ways I've found to handle this, and it depends entirely on the client.
the first is to scope the account management time into each project or retainer behind the scenes, without showing it as a separate line item. This works well for smaller clients or anyone running just a single channel. The cost is just baked into the overall fee, so there's no awkward conversation about "why am I paying for someone to manage my campaigns?"
The second approach is to scope the same time, but present it as a visible line item in the quote. this suits larger clients or multi-channel campaigns, where they already expect to pay for a project manager or account manager. having it visible actually gives them confidence that someone is co-ordinating everything - and if you're managing three channels, that's easy to justify.
what worked for us was a mix of both, depending on the client. The AM team had their own direct fees from the clients where we itemised, plus a share of fees transferred from each channel team where we didn't. that let us keep a clear view of salary versus revenue for the whole account management team.
Whichever route you take, you absolutely must scope the expected AM time per client and then deliver within that scope. Especially if you're using the first approach, where time and fees are being transferred from other teams rather than sitting as a contract line item. the last thing you want is your scoping doc saying the client is paying for five days of SEO, the SEO team delivers five days, and forgets a chunk of that time was meant to be allocated to management. that's how you bleed profitability without realising it.