For open-access journals, articles are freely available online without any restriction. Therefore, it is possible to cover the cost of the whole publishing services provided by publishers, i.e., article process charges (APC).
The process may include provision of online tools for editors and authors, article production and hosting, liaison with abstracting and indexing services, and customer services. The APC, payable when your manuscript is editorially accepted and before publication, is charged to either you, or your funder, institution or employer.
A waive of APC or discounted APC policy is also carried out by SMP journals upon authors’ request.