It is not possible to create top-up payment from CCP v2 after upgrade to OSA 7.3. An error message appears after some time of waiting:
Unable to complete the operation, please cancel and try again later.
Xmlrpcd is setup to work over http, not https (SslPrivateKey and SslCertificate are commented in /usr/local/bm/etc/ssm.conf.d/xmlrpcd.conf). The bug with id #PBA-83568 ("Top-Up payments do not work from CCPv2 after upgrade to 7.3") has been created about payment management aps sending API calls over https without any checks.
Setup xmlrpcd to work over https.