Hi Christian,
As per Industry Best practice, its always recommended to use the SAP standard clearing reasons.
The reason being there are Standard SAP checks based on the clearing reason in many business processes.
So I would not recommend you to go for a custom clearing reason.
What to Do?
I guess you will have a unique identifier in the Payment lot to identify which one is Pay pal and which one is for credit Card etc. It can be document type, bank clearing A/c etc..
First Create three clearing categories and three clearing variants, say one for Pay pal, one for CC and one for Bank Statement.
Now assign them as the alternate clearing variant for the clearing type 05 (payment Lot).
So you will have three different clearing categories with three different clearing variants.
Now make a copy of the FM, FKK_SAMPLE_TFK113E_04 and plug in an enhancement based on the unique identifier whether its for pay pal or credit card etc, and change the Clearing category.
Assign this custom FM to "Program Enhancement" under event 04 in the "Define Defaults for Incoming Payments" configuration node.
In this event, you can overwrite the clearing category that was determined by the program in the standard system that is being maintained at the CA level.
So now depending on the unique identifier, the event will set the clearing category, which in turn will trigger the clearing variant (for paypal or credit card) and will clear the outstanding receivables accordingly.
Hope it helps.
Thanks,
Amlan