Marlene, there are different varities of ProMix. I think most people use the Promix BX for wintersowing purposes. You might want to read some reviews from veteran gardeners tell their experiences..Joannabanana over at Dave's Garden and Cubits.org
and Karen(Kqcrna) post on those two websites as well as gardenweb.com..There are many others that choose to use Promix products. Home Depot and Lowe's in our area only uses Miracle Grow. Since this is your first year trying wintersowing, why not save yourself some money, and buy Miracle Grow and doctor it up. Despite what people might say, buying seeds and soil, and depending on what type of containers you have chosen to use..it takes a bit to get started. Next year when you feel more comfortable, you can invest in the more expensive stuff. Just a suggestion. If you can d-mail me, I'll give you a list of the places in our area that sell Pro-mix products and what they prices I was quoted back in January.