  With the growth of open access (OA), the financial flows in scholarly journal
publishing have become increasingly complex, but comprehensive data and
transparency into these flows are still lacking. The opaqueness is especially
concerning for hybrid OA, where subscription-based journals publish individual
articles as OA if an optional fee is paid. This study addresses the lack of
transparency by leveraging Elsevier article metadata and provides the first
publisher-level study of hybrid OA uptake and invoicing. Our results show that
Elsevier's hybrid OA uptake has grown steadily but slowly from 2015-2019,
doubling the number of hybrid OA articles published per year and increasing the
share of OA articles in Elsevier's hybrid journals from 2.6% to 3.7% of all
articles. Further, we find that most hybrid OA articles were invoiced directly
to authors, followed by articles invoiced through agreements with research
funders, institutions, or consortia, with only a few funding bodies driving
hybrid OA uptake. As such, our findings point to the role of publishing
agreements and OA policies in hybrid OA publishing. Our results further
demonstrate the value of publisher-provided metadata to improve the
transparency in scholarly publishing by linking invoicing data to
bibliometrics.
