What the fee is, and when it falls due
In the open-access model the reader pays nothing and the cost of publication is met on the author’s side, usually after acceptance. In the subscription model the reader’s institution pays and there is typically no author charge, though some journals levy separate page or colour-figure charges. Hybrid journals are subscription titles that offer an optional open-access route for a fee on a per-article basis.
The normal sequence is: submit, review, acceptance, then an invoice. Being asked for a substantial payment before peer review has happened is unusual and worth questioning — a small, clearly published submission fee exists in a few fields and is a different thing from a charge that appears once you are committed.
Fees are set per journal, not per publisher, and are revised annually. Read the figure on the journal’s own fee page on the day you decide, not a number quoted in a list article — including this one, which is why no amounts appear here.
A published fee on the journal’s own site, payable after acceptance, is normal. A fee that appears only after acceptance is not.
You may not have to pay it yourself
Check your institution first. Many universities and consortia hold transformative or read-and-publish agreements, under which the institution’s payment covers both subscription access and open-access publishing for its corresponding authors in that publisher’s journals. Where one applies, the charge may be fully covered, and authors regularly pay fees they did not owe because nobody checked.
Check the funder second. Many research funders permit publication costs as a direct grant cost, and some mandate open access and provide a budget for it. The rule is usually specific about which routes qualify.
Check the waiver policy third. Most major publishers operate automatic or discretionary waivers and discounts for authors in lower-income countries, and some journals waive fees on request in cases of demonstrated need. These policies are published, and applying for one is routine rather than exceptional.
The green route costs nothing
Open access is not only a fee. Under the green route you publish in a subscription journal at no charge and deposit a permitted version of the manuscript — usually the accepted manuscript, after peer review but before typesetting — in an institutional or subject repository.
What is permitted, and after what embargo, is set by the journal’s self-archiving policy. Publishers state these terms, and services such as Sherpa Romeo collate them. The deposited version is often what makes a paper findable and citable for readers without subscription access.
For authors without funding this route is frequently the right answer. It satisfies many open-access mandates, costs nothing, and does not require choosing a journal on the basis of what you can afford.
When a fee is a warning sign
Charging an APC does not make a journal predatory — many entirely reputable open-access journals charge substantial ones. What distinguishes a problem journal is concealment and timing: a fee that was not findable before submission, an invoice for services never mentioned, or a charge attached to an acceptance that arrived within days of submission and contained no substantive review.
Watch for fees attached to things that should not carry them. Charges to withdraw a manuscript, to receive a reviewer report, or to have a paper "fast-tracked" past review are not features of legitimate publishing.
The safest habit is simple: find the fee page before you submit, and screenshot it. If the number you are invoiced does not match the number that was published, you have grounds to query it — and a record.