Apart from the 29$/month for the basic website, are there any other additional charges? for example, designing and building the website.
What is great about shopify is the amount of free and premium themes they have that are responsive. In my quick start guide you get by subscribing to my email list I talk about creating a minimum viable product.
This is getting your product up as soon as possible so you can start getting data about what works. For starters don’t get a designer to design your site. Find a theme similar to what you want. You can always make it better later… But for now just get something up.