Is it possible to use my in-store credit card processor with my Shopify website?
Yes, absolutely! Shopify currently supports third-party gateways from: PayPal, Stripe, Authorize.Net, FirstData, Braintree and Paymill.
Modern Retail typically uses Authorize.Net and configures it to use your in-store credit card processor. Authorize.Net is not a credit card processing company but, rather, is the company making the connection between Shopify and your credit card processing company. Please read the following article for more information:
Modern Retail will set up and configure both Shopify and Authorize.Net to work with your new website. However, if you're interested, here's some additional information about Shopify's supported gateways and configuring Aurthorize.Net:
If you have any questions, please create a Support Request. We'll be happy to help. Thank you.