Пример исходных данных: 
bill:issuer = "sas@test.test"
buyer = "3283005053463"
currency = 51
sum = 1.0
description = "Test"
issuer_id = "issuer0023"
valid_days = 1
Пример отправляемых данных:
bill:issuer = URLEncode("sas@test.test") = sas%40test.test
buyer = 3283005053463
currency = 51
sum = 1
description = URLEncode (Base64("Test")) = VGVzdA%3D%3D
issuer_id = URLEncode (Base64("issuer0023")) = aXNzdWVyMDAyMw%3D%3D
valid_days = 1
checksum = md5("kl3sDvf67*h&#TVFcD=Ghef987UnYDFhufD%&DFkJhjkBDv7!DS68LMdFnmWE&F*&Tdvc6gdjH23grfd7GbHkdJhgKd" + "sas@test.test" + "3283005053463" + "51" + "1.0" + "VGVzdA==" + "1" + "aXNzdWVyMDAyMw==") = 15dc50a6870bdfeecd670899aeb22c4e
Итоговое тело запроса:
description=VGVzdA%3D%3D&sum=1.0¤cy=51&issuer_id=aXNzdWVyMDAyMw%3D%3D&valid_days=1&bill:issuer=sas%40test.test&buyer=3283005053463&checksum=15dc50a6870bdfeecd670899aeb22c4e