You can download API and SCI documentation here:

DescriptionVersionAuthorpublishedDownload
S-PAY XML API Developer Documentation
2.02S-PAY Team2015/07/23
API 2.01 PHP Test Script2.02S-PAY Team2015/07/23
S-PAY SCI Developer Documentation
2.00S-PAY Team2015/07/20
S-PAY SCI Button Generator und Call by Link Documentation2.00S-PAY Team2015/07/20