This help page lists the payment services E-junkie supports, any of which can be used for digital products:
If you are selling downloads, while we do log the number of download Attempts and last download IP for each buyer (which you can find in your E-junkie Transaction Log), unfortunately it is impossible to tell whether any browser-based download Attempt ran to completion or resulted in a valid (non-garbled) file, let alone prove that any complete and valid file satisfied the buyer's expectation of what they would receive for their purchase. As such, payment reversals/refunds are just a "cost of doing business" when selling downloads delivered via the Web.
The only conceivable way around that would be to have a "closed ecosystem" where you completely control end-to-end the servers and apps used to purchase, download, and use the files you're delivering; this is something E-junkie cannot provide.
If you are selling serial/license/registration codes for a gaming platform, your E-junkie Transaction Log will list the code we issued to each buyer; if they complain they didn't get it, you can simply copy it from your log and paste it into a personal reply. If the gaming platform is your own, you should have some means to tell when a code has been redeemed in your system, which would prove that the code was both delivered and used, and of course you could deactivate or revoke the code at your end if the buyer disputes their purchase of it.