I understand.
The error on line 1203 is caused by WordPress not accepting the .csv file.
Are you using an administrator account to upload the transactions? If not, it might be worth a shot.
If all else fails, the option of setting ALLOW_UNFILTERED_UPLOADS is not a huge risk in my opinion, if you only have it set for the few seconds that it takes to upload the file. An attacker would have to use that time to set to upload some executable file, then use some other vulnerability to run it and somehow gain access.
It’s your call.