It doesn't say so explicitly in the article, but a common source of underpricing is the assumption that the difference between the cost to make and the selling price is your profit. Many people forget that they are in business to make a living and you have to include in your pricing a wage to yourself.
I'm lucky in that what I sell in this shop comes in fairly standard sizes. Even so there are several options for a given listing so I've just spent a day setting up a spreadsheet that helps me calculate the cost of production and set a selling price.
I have included the cost of all the components of my products, estimated the time I take to make them and on the basis of an hourly wage rate added the cost of manufacture/assembly. On top of that I have added a % for miscellaneous overheads. That's currently pulled out of the air but is easily changed as a variable. On top of THAT, I've added my profit margin. It is vital that you don't confuse the wage you pay yourself with the profit. The wage goes on personal living expenses - food, rent etc. Business overheads like travel, rent if any, heating, materials etc are separate. Only after all that is calculated can you set a profit margin.It is from the PROFIT and the surplus that you accumulate that you grow and expand your business.
Remember - if you don't cover all your costs, then essentially you are paying your customers to take your goods away!
Of course it may take a year or so to get into profit as you grow sales,but that is a separate issue. Even if you feel you can't actually pay yourself a living wage yet, build it in, then you have a benchmark against which to measure how well you are doing.