If I were you I try the following (assuming you know which stores you want):
1. Play dumb and call their support number and when they answer tell them that you dont know why you were routed there when you wanted to talk to the marketing department.
2. Create an email blast to either firstname.lastname@example.org or email@example.com chances are if it is a small operation the email will be seen by one of the founders/owners and they might take a peek at your offer.
3. Spend a few bucks and buy InMail contacts with Linkedin so you may reach the right person of the company you are targeting.
Or you could spend some big bucks on a tech blog site like techcrunch or get a blogger from a portal like http://www.ppchero.com/ to write a post on your offering
If you can afford it buy a list of ecommerce companies. If you want to stay lean source your own list. InternetRetailer.com is likely a good place to start.
Also data mine LinkedIn and use Inmails or tools like https://salesloft.com/ to contact the marketing leads at ecommerce companies.
I would segment my data mining activities based on your perceived sweet spot. Look at the ecommerce companies by their:
- number of employees
- geographic location
Reach out to them via LinkedIn and email with some specifics. Look at their site, current ppc efforts, etc. Give them some free insights.
Good Luck! I'm happy to hop on a Clarity to call to go deeper here.