Hi Renae,
I wanted to address your question...which often comes up here on the site and was explored in a book I reviewed here entitled,
'The Wealthy Freelancer'. I highly recommend you pick up a copy as it is a great resource.
If you do not know how long a particular task is going to take you can do a few things:
Do a test run and time yourself. So in your SOC situation, you might have given yourself a half hour to process the work and then decide based on volume you were able to achieve, decide upon a packaged rate to offer the client. So if you need to make $XX per hour, and you processed 50 SOC, decide upon your packaged rate on that calculation.
Another option is to offer a packaged rate, $XXX to finish the project in a specified period of time and then if the project goes over that amount of time then charge your hourly rate.
You would need to explain to the client that there are 'unknowns' in this performing this task. Sometimes services are not so 'cut and dry'.
I would suggest, if you do not already...take a deposit for at least 50% of the projected fee in the event the client does not pay you for the additional time you spend working on this task. I would also suggest, that you haven't already, do include your payment terms in the signed contract so that it is clearly understood what you have decided to do.
In the future, should you offer this service, you will know what to charge as a package having done this before.
Good Luck! Let us know how you do.
Janine