Honest opinion
I personally think it was the wrong thing to do and i would suggest that you withdraw your proposal. But i don't think your job will hire your company to do any work as long as your employed there. it can create a conflict of interest, not only for them but for you too.
if your currently working your job and they have an issue, they might question why they have to pay your invoice when they are paying your wages.
reselling items and equipment from your own business to your job company could get very questionable. will they see that your reselling around the market rate or are they highly inflated because your telling them to buy from your business as an employee.
Because your also an employee, your may expect more from you as a company, or a discounted rate for hiring you.
Overall, i would suggest one of two things after you withdraw your contract proposal. And better yet, you could offer the choices to your employer/manager.
1. You stop working on their computer/network issues and you continue to with your normal job duties.
2. You request a raise in pay for the additional job duties you would be talking on. As it might make more sense to have a tech in house that to problem solve issues as they occur.
They are going to wonder why they should pay you a few dollars more an hour, when they wont need your skills all the time, and they could just pay another tech to come in and fix the issue for a flat fee. Well if it is just you, then it makes sense to continue with the other tech, but in a bigger office with more employees, it would create more down time that would cost the company more money.
It was a right move. At least they should give you a raise or something if nothing else. They are being stupid.
I wouldn't expect the employer to approach me with the option of a raise, they might request that you fix the issues though. The employers job is to get the work done efficently for the least amount of money possible. The employee's job is to get the job done for the most amount of money. the issue is that most employees don't see this concept and need to think of themselves as a business, and therefore never request to be compensated for extra work. "Why buy the cow when you can get the milk for free?"