As you decide which way to go in your e-commerce web design, there are three things you need to consider: features, budget and time.
The features you need for your e-commerce business is the biggest factor in deciding whether to use an open-sourced platform or to build your own custom solution.
Advantages of open-sourced systems like Woocommerce and Shopify are their ability to handle bulk visitors and big transactions, easy integration with other systems, and consistency of uniformed appearance throughout the website. They also have a pre-built baseline framework, with plenty of third-party plug-ins and extensions, most of which are free of charge.
For products that require complex configurations and checkout processes, you may want to consult with a web designer to verify if open-sourced sites can support your requirements or if you need a tailor-made site to better suit your users.
You also need to factor in the time spent tweaking an existing e-commerce platform versus building something from scratch. Sometimes, it is easier to design your own website rather than working around existing solutions.
One of the benefits of designing your own online store is that web designers can consistently use the latest web code practices and the latest SEO techniques to ensure your brand is optimized. A major factor in creating your website is owning the code, so you can freely move your shopping cart to another developer or hosting plan when the need arises.
If you have a limited budget, it is best to keep your site more simple at the start so you can begin generating sales as soon as possible.
On another note, as your business grows and you add more products to your online store, you are better off using open-sourced platforms because they are easier to maintain.
If you need to launch your website asap, go for pre-built solutions, which offer the usual e-commerce web design workflow: products -> product -> cart -> checkout -> order. For a simpler process, you can create a quick single-page form.
When you are not in a rush and you are willing to wait for a custom-designed e-commerce platform, think of the features you want for your online shop. You can look at examples of e-commerce sites you like online and apply e-commerce best-practices on your site.
In the end, it all boils down to your own preferences, budget, timeline, and requirements to determine how you will set up your e-commerce website and how your customers will use it.