a sale is a sale,whether its over the counter or on a website.
you don't need two separate entities, however if there are significant diffenrecs you may need three tables usign the sub/supertype approach. that you be one table for the common items, and two sub / child tables for whats different between the two. ferinstance taht could be things like delivery address, invoice address, email for the web sale.
plenty would argue that unless you have some significant diferences a single table woudl suffice.
its a quite a common approach and its mirrored by the real world. OK its not the same but
many shops only sell on account administratively so any sales over the counter are assigned to an account. if the customer doesn't have an account then the organisation registers those sales to a generic account called cash account or counter sales / whatever. in principle your requirement is similar
I'd rather be riding on the Tiger 800 or the Norton