He said that "unfortunately" people living in parts of the Highlands and Islands were having to pay more than those living in other parts of the UK, even after principles aimed at implementing a fair pricing structure have been unveiled.
Mr Ewing called on companies to sign up to the statement of principles, which was developed by the Parcel Delivery Working Group last year.
These ask businesses to avoid charging disproportionate delivery costs, provide greater transparency for consumers and deliver to the widest possible area.
Mr Ewing said: "At this time of year shoppers are already starting to think about ordering online and getting in early with Christmas shopping. Unfortunately we're still finding that many customers in Scotland's Highlands and Islands are faced with disproportionate costs when it comes to the delivery of their online purchases.
"It's not fair that people in island communities or the Highlands are being charged an excessive amount when they are ordering the same items as people in other parts of the UK.
"But it's not just customers who are affected by postcode penalties, businesses in rural areas are also hit hard by delivery surcharges.
"Retailers should take on board our statement of principles and in turn customers, particularly in the Highlands and Islands, will receive a better service."
Research published by Citizens Advice Scotland earlier this year suggested that an estimated 15,000 firms are having to deal with punitive extra fees.
Some 90 per cent of the near 250 businesses who responded to a survey reported they are charged extra for having items delivered just because of their postcode and 76 per cent felt this had a substantial impact on their business.
About eight out of 10 regularly encountered misleading claims of "free delivery", when surcharges were actually imposed due to their location.
More than three-quarters of the Scottish respondents were regularly classed as "offshore" or "remote" when ordering items online. This included businesses whose premises were on the Scottish mainland but were considered an island.
The voluntary guidelines were drawn up last year after discussions with the industry and the Scottish Government.
The UK Government then followed suit by issuing its own guidance last month, developed by Citizens Advice and the British Retail Consortium.
Mr Ewing said: "By abiding by the guidelines, online retailers can show respect for customers in all parts of the UK."