Very simple. Some eyeglasses shops online are the manufacturer.
Since eyeglasses shops online are bringing discount eyeglasses from the factory directly to you, via the internet, they don’t have the additional overhead constraints and expenses that the traditional non-discount eyeglass retailer will have. Eyeglasses shop online have eliminated distributor and wholesaler mark-ups on their whole line of discounted eyeglasses along with the costs of retail space, behind-the-counter employees, and warehousing. Thus the steep discount is passed on to you, the general public. That is why eyeglasses shop online can offer your cheap glasses, and prescription glasses.