If you successfully register your invoice number with DLsite in any month following its issuance, the invoice system wholesale price will apply to applicable sales from the month of registration on DLsite.
In the case of a late application to DLsite, retroactive payments for past months with invoice system wholesale prices cannot be made.
Please read the "Changes to the Invoice System" page for further details.
For international circles residing outside Japan:
https://www.dlsite.com/home/guide/invoice-international
For circles in Japan:
https://www.dlsite.com/home/guide/invoice
https://www.dlsite.com/home/guide/invoice#block10