ACQUISITIONS - Preliminary Purchase Consideration of the Acquired Assets and Assumed Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Sep. 30, 2021	Jun. 30, 2021
Business Acquisition [Line Items]
Goodwill	$ 353,882	$ 292,243	$ 289,046
BIS
Business Acquisition [Line Items]
Accounts receivable		1,567
Settlement assets		6,889
Inventories		458
Prepaid expenses and other current assets		10
Property and equipment		206
Goodwill		46,660
Operating lease right-of-use assets		0
Other assets		0
Total assets acquired		104,090
Accrued expenses and other current liabilities		138
Settlement obligations		6,889
Deferred revenue, current		1,568
Current portion of operating lease liabilities		0
Operating lease liabilities, less current portion		0
Net assets acquired		95,495
BIS | Capitalized software
Business Acquisition [Line Items]
Intangible assets		15,200
BIS | Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets		32,300
BIS | Non-compete agreements
Business Acquisition [Line Items]
Intangible assets		100
BIS | Trade name
Business Acquisition [Line Items]
Intangible assets		700
ImageSoft, Inc.
Business Acquisition [Line Items]
Accounts receivable		4,997
Settlement assets		120
Inventories		0
Prepaid expenses and other current assets		2,897
Property and equipment		433
Goodwill		22,408
Operating lease right-of-use assets		332
Other assets		6
Total assets acquired		54,403
Accrued expenses and other current liabilities		910
Settlement obligations		120
Deferred revenue, current		6,748
Current portion of operating lease liabilities		75
Operating lease liabilities, less current portion		250
Net assets acquired		46,300
ImageSoft, Inc. | Capitalized software
Business Acquisition [Line Items]
Intangible assets		5,200
ImageSoft, Inc. | Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets		16,300
ImageSoft, Inc. | Non-compete agreements
Business Acquisition [Line Items]
Intangible assets		610
ImageSoft, Inc. | Trade name
Business Acquisition [Line Items]
Intangible assets		1,100
Other
Business Acquisition [Line Items]
Accounts receivable		3,141
Settlement assets		0
Inventories		161
Prepaid expenses and other current assets		2,043
Property and equipment		312
Goodwill		35,955
Operating lease right-of-use assets		484
Other assets		32
Total assets acquired		71,498
Accrued expenses and other current liabilities		1
Settlement obligations		0
Deferred revenue, current		5,498
Current portion of operating lease liabilities		221
Operating lease liabilities, less current portion		251
Net assets acquired		65,527
Other | Capitalized software
Business Acquisition [Line Items]
Intangible assets		4,100
Other | Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets		24,040
Other | Non-compete agreements
Business Acquisition [Line Items]
Intangible assets		390
Other | Trade name
Business Acquisition [Line Items]
Intangible assets		840
2022 Business Combinations
Business Acquisition [Line Items]
Accounts receivable	678
Settlement assets	685
Prepaid expenses and other current assets	83
Property and equipment	190
Goodwill	61,590
Operating lease right-of-use assets	263
Other assets	22
Total assets acquired	115,941
Accrued expenses and other current liabilities	314
Settlement obligations	685
Deferred revenue, current	30
Current portion of operating lease liabilities	82
Operating lease liabilities, less current portion	181
Other long-term liabilities	6,968
Net assets acquired	107,681
2022 Business Combinations | Capitalized software
Business Acquisition [Line Items]
Intangible assets	9,790
2022 Business Combinations | Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets	41,090
2022 Business Combinations | Trade name
Business Acquisition [Line Items]
Intangible assets	$ 1,550
2021 Business Combinations
Business Acquisition [Line Items]
Accounts receivable		9,705
Settlement assets		7,009
Inventories		619
Prepaid expenses and other current assets		4,950
Property and equipment		951
Goodwill		105,023
Operating lease right-of-use assets		816
Other assets		38
Total assets acquired		229,991
Accrued expenses and other current liabilities		1,049
Settlement obligations		7,009
Deferred revenue, current		13,814
Current portion of operating lease liabilities		296
Operating lease liabilities, less current portion		501
Net assets acquired		207,322
2021 Business Combinations | Capitalized software
Business Acquisition [Line Items]
Intangible assets		24,500
2021 Business Combinations | Acquired merchant relationships
Business Acquisition [Line Items]
Intangible assets		72,640
2021 Business Combinations | Non-compete agreements
Business Acquisition [Line Items]
Intangible assets		1,100
2021 Business Combinations | Trade name
Business Acquisition [Line Items]
Intangible assets		$ 2,640