My main source of income for day-to-day operations include just simply fixing computers or doing upgrades, which usually bill the client less then $200 when all is said and done. For this small of a charge (multiple clients per day, mind you), is it worthwhile having a contract and full Scope of Work drawn up? Is it OK to just use a work order and invoice? Or am I opening myself up to bigger issues?
Work orders and invoices are contracts. Documents detailing the agreed upon work to be done and payment for that work.
Work orders define the scope of work and the invoice defines the terms of payment. You should have costs on the work order if you don't already. Listing your liability on work orders and dispute resolution terms on invoices is a good idea as well. If the client agrees to the work order, they've expressed agreement to the terms.
Just because something doesn't have the word "Contract" on it, it doesn't mean it's not a contract.
As Scott says, your work order is a contract and should probably include the scope of work, the payment and payment terms.
When I was doing this type of work years ago, I would not start on a job until the client signed a form agreeing that they are responsible for their own data, that they have a current backup and that I cannot be held responsible for any data loss. This is important to avoid being sued for any data loss, whatever the reason. Data loss can happen through hardware failure or malware or even your own mistakes but ideally the client should have a backup to be able to restore the data.
You might also get clients to sign a document stating that they own appropriate licences for the software they are asking you to install on their computers. You don't want to be held responsible for installing unlicensed software on a machine when you have been led to believe there is a legitimate license.
I would not dig myself into administrative things for such a small money and such a large number of clients. I sign contracts only where there is a good and long-term job where I want to bind the client not to make silly moves (like leaving me all of a sudden).
Of course, the most important thing is what your Laws say. If they do not prescribe having contracts for such small jobs, then I would not complicate my life.