Why the corridor has no single price
Ethiopia has not joined the Hague Apostille Convention, so a British mark alone does not answer for a paper used there. Files heading that way need embassy or consular legalisation instead: certification by the FCDO Legalisation Office, then a consular stamp on top of it. Two bodies, two bills, and only one of them written into a British price list.
Where Ethiopia stands
An apostille is not accepted for Ethiopia. That one line reshapes a budget more than anything else on this page, because it moves the finishing act out of Britain altogether. A plan built on the published figure alone will be short by whatever the second desk asks, and the shortfall shows up late, when the file is already halfway to where it is going.
What each payment buys
The published British figure buys one thing: certification by the FCDO Legalisation Office on one document. It is counted per sheet and not per envelope, so a file holding four records is four charges even though it posts as a single packet. Counting in envelopes rather than in sheets is where people under-count, and they do it more here than anywhere else on the run.
The consular payment buys the act the destination actually reads. Its size, the way it is paid and the pace behind it are settled at that desk, not in Nottingham. Nothing done at the British end moves it, and no British receipt covers it, so it belongs in your plan as a line of its own rather than as a rounding allowance tacked on the end.
Building a figure you can trust
Count the sheets, apply the published rate, and treat that total as the part you know. Above it, leave a margin for the consular stage that feels generous rather than neat. A budget that only holds together if the unknown half turns out to be small is not a budget at all, and this is the one route where that shows up quickly.