We were planning to buy a new theme from Shopify and get started with their monthly program. However, in the nick of time, we discovered that they require us to pay their merchant card fees, which were 2.9% plus 30 cents per transaction. This is a high rate, one I didn't want to pay, since we now pay 1.9% with Chase.
So we asked if we can use our merchant card processor. YES we can, BUT they will charge us an ADDITIONAL 2% per transaction if we do that. This is just plain hardball, forcing you to pay their high rates. We don't do business with companies that use unethical business practices, so this ended the possibility for us.
Be careful if you are considering this company. We saved thousands of dollars per year by getting the lower merchant card rate. You will lose a lot by using this service.