After reading several threads, I've found that most don't use software much. Many use old fashion paper and pencil/ink. I also haven't seen many mention 'custom' shares. I know from my experience custom shares can get to be a PIA. Charging to credit cards are you own personal preference. My son uses the paypal way and has been happy with it. Some of his friends have use "Square" and were happy until they got stung by customers disputing sales and the credit card took the customers side, so the vendor was out the item and the money. I haven't had used any so far.

Nope, never heard about any CSA and Ecommerce Software.

Can anyone please tell me why are they used for? And which one is the Best?

You can simply start selling goods online https://elogic.co/blog/how-to-sell-food-online/