If you need to make a basic ecommerce site, then it might be viable to look at an existing ecommerce platform such as Woocommerce on Wordpress. For a basic store it is free to set up and you can do it really quickly. You can also do stock management, integrate with third-party payment providers such as Stripe easily and the client can manage their store without you needing to add the products manually. From experience, this is definitely the easier option for a simple store that doesn't need anything fancy.

However, to answer your question about online payments: when it comes to online payment forms in your own application, it's generally best to let a third-party payment provider facilitate the payment such as Stripe, PayPal (or PayFast here in South Africa). They take care of all the security around storing card details and making the actual transaction and only take a small percentage of the total payment as commission for facilitating the process. They not only make it less work for you to integrate payments into your application, but they are also compliant with various regulations on how financial information should be stored (e.g. credit card details) and so it'll save you that trouble as well. Plus, the user will be more familiar with Stripe/PayPal interfaces so you probably won't need to add any additional instructions for the payment.