Airbnb now offers guests to pay in installments. If a guest selects that option (and most do- why would anyone pay everything upfront if they do not have to?), you cannot bill them for anything extra until the second installment is paid. I believe it is due about 2 weeks prior to arrival. So first off, you have nothing to worry about in terms of your guest not paying for the reservation. The reservation is safe. As for the extra, just tell the guest you will invoice them when they pay for the main reservation per Airbnb policy. I usually ask mine to message me once they pay but they rarely remember to do it. I go over my communication with the guests when I get a reminder from Airbnb that the guest is coming which is sent about 8 days prior to arrival. At that point everything is paid and I can invoice them for the extras they wanted. It is all extremely inconvenient. We should be able to “pile” our new invoices on top of whatever else is outstanding. I would however imagine this is the lowest priority for Airbnb. They will fix this right after they are done perfecting the extenuating circumstance policy and host guarantee. Oh, wait, they think they have already perfected it. Never mind, this will never be fixed.
I am not 100% sure but it sounds like the guests has paid in part for their stay. Airbnb allow guests to pay part when they book and the rest a few weeks later, or if multiple guests are staying for each guest to pay their share of the cost individually. Once the basic fee has been paid in full you can then take the payment for extras. That is my understanding perhaps someone else can further clarify this from their experience.