As was already stated, having an SSL of your own for a secure order form is crucial for any e-commerce website. Security is a HUGE business online and there is only a small percentage of people online that would order without a secure connection ... and a portion of those are most likely unaware that it is insecure.
I have worked with a few e-commerce websites and the smallest bit of doubt that the form is secure will cause someone to go to a different website. Whether that is using the free SSL certificate provided by your host instead of buying one for your domain or even having an unsecure image reference on your page ... they will run fast.
Like others said, $100 / yr is minimal for any business expense ... especially for an online shop where a single sale could cover that cost easily (and guarantee you losing customers without it). The fact of business is you need to spend money to make money. If you are selling clothing, I'm guessing you must have had to buy some different sizes to have in stock before selling anything? It's just part of running your business. If you are getting a lot of traffic, it's only a matter of time that you will start to see a return.
On a side note, you may want to check to make sure you are targeting the right visitors and sending them to the best landing page too. I don't know your campaigns, but sending them to the best page for them to find what they are looking for is going to be crucial for converting that sale as well.