I have heard about people sending in, via email or upload, their image to a company and then the company makes the transfer for you and all you have to do is press it yourself. Is there anyone you can recommend?

I recommend me

I noticed on your website that you use vinyl transfers. What is the difference between vinyl heat transfers and plastisol heat transfers?

You're limited with vinyl. You can't do real fine detail with a plotter and heat transfer vinyl. Nor can you do halftones and you are limited to number of colors and color in general. With plastisol transfers you can do so much more. Plus, for large runs it's very time consuming weeding vinyl.