Yes, you need a valid GST number if you want to sell online on any e-commerce platform. You will need to provide a valid GST invoice whenever a customer purchases your product. Currently, we are using Zubizi GST accounting software to manage all our GST related work. Using it is very easy. It has a super easy invoicing system. You can Generate an invoice with few clicks. They have a free trial on their website. And pricing is very affordable. Visit their website for more info.