SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Disaggregated Revenue (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021		Jun. 30, 2022	Jun. 30, 2021
Concentration Risk [Line Items]
Total revenue	$ 80,553	$ 63,129	[1]	$ 232,612	$ 156,947	[1]
Revenue earned over time
Concentration Risk [Line Items]
Total revenue	59,584	43,735		170,339	112,052
Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	20,969	19,394		62,273	44,895
Merchant Services
Concentration Risk [Line Items]
Total revenue	32,714	29,963		91,071	81,130
Merchant Services | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	25,643	22,617		70,976	60,100
Merchant Services | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	7,071	7,346		20,095	21,030
Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	47,839	33,729		141,575	77,491
Proprietary Software and Payments | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	33,951	21,638		99,390	53,492
Proprietary Software and Payments | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	13,888	12,091		42,185	23,999
Other
Concentration Risk [Line Items]
Total revenue	0	(563)		(34)	(1,674)
Other | Revenue earned over time
Concentration Risk [Line Items]
Total revenue	(10)	(520)		(27)	(1,540)
Other | Revenue earned at a point in time
Concentration Risk [Line Items]
Total revenue	10	(43)		(7)	(134)
Software and related services revenue
Concentration Risk [Line Items]
Total revenue	39,006	26,819		114,316	61,362
Software and related services revenue | Merchant Services
Concentration Risk [Line Items]
Total revenue	3,349	2,975		9,319	8,716
Software and related services revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	35,667	23,852		105,023	52,654
Software and related services revenue | Other
Concentration Risk [Line Items]
Total revenue	(10)	(8)		(26)	(8)
Payments revenue
Concentration Risk [Line Items]
Total revenue	36,683	32,223		104,677	86,171
Payments revenue | Merchant Services
Concentration Risk [Line Items]
Total revenue	27,135	24,995		75,365	67,051
Payments revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	9,559	7,770		29,341	20,756
Payments revenue | Other
Concentration Risk [Line Items]
Total revenue	(11)	(542)		(29)	(1,636)
Other revenue
Concentration Risk [Line Items]
Total revenue	4,864	4,087		13,619	9,414
Other revenue | Merchant Services
Concentration Risk [Line Items]
Total revenue	2,230	1,993		6,387	5,363
Other revenue | Proprietary Software and Payments
Concentration Risk [Line Items]
Total revenue	2,613	2,107		7,211	4,081
Other revenue | Other
Concentration Risk [Line Items]
Total revenue	$ 21	$ (13)		$ 21	$ (30)

[1] Effective October 1, 2020, the Company's financial statements are presented in accordance with ASU 2021-08, Accounting Standards Codification Topic 805, Accounting for Contract Assets and Contract Liabilities from Contracts with Customers. See Note 2 to the interim consolidated financial statements for a description of the recently adopted accounting pronouncement and the impacts of adoption on the condensed consolidated statements of operations.