The model is a good simplified SaaS model, which is where early stage companies should start. Then measure and iterate until the variables are nailed down.
The GAAP nerd in me wants to point out that merchant fees should not be in COGS. It is a variable cost, but it is cost to collect, not a cost to sell.
Many companies keep Gross Margin according to GAAP and also report on a Contribution Margin to capture all the variable costs. This will provide a percentage that every dollar of revenue contributes (get it) to overhead.
Thanks for posting.