Invoice Simple is a mobile-first invoicing product. It runs on iOS and Android (and a web app), has built-in payment processing through Stripe and PayPal, stores a customer database, supports recurring invoices, and is priced around $9 to $40 a month depending on which tier you pick. It is built for people whose business runs on invoicing, not for occasional users.
meraSaaS includes an invoice generator but it is not the whole product. The invoice tool is one of 26 tools, and it is free to use. It produces a clean invoice PDF from a form, with country-aware tax presets and browser-side processing. There is no payment processing, no customer database, and no recurring invoices, though you can optionally save invoices to your workspace after signing in.
These are different shapes of product for different people. Here is the actual difference.